This PR is based on the develop branch and it implements the following changes:

• restore support for an external folder using scoped storage permissions via SimpleStorage library. Note that due to how permissions now work, the filetype defines the file extension, so that all files now are .1list.json instead of .1list as before (EDIT: in fact it's possible to force a custom file extension by not specifying a mimeType in makeFile() in PersistenceHelper.kt, but then it may be risky in the future, maybe Android may confuse this with a MediaStore file and restrict permission if it is unsure it's a DocumentFile, so I would rather keep .1list.json just to be sure, and also 3rd-party reading apps can more easily open these files). Big thanks to @anggrayudi for his help with making this possible despite Google's insanity! (See issues here and here). Fixes Error while getting your list from content #41, fixes Default storage folder is ignored #26 and fixes Export/Import lists #38.
• a Share All Lists button as a quick workaround to export a backup of all lists easily.
• option for Share and Share All export in Markdown or plain text.
• option to add new tasks at the list's end instead of top. Fixes Let user decide if items are added to top or bottom of list #32.
• option to NOT move done tasks. Fixes [Feature] Optional reordering when done #10 and fixes Please let the order of the list-items as they are #27.
• option when editing a task to move it to another list. A typical use case is to move urgent tasks to an "URGENT" list, and when done or not urgent anymore, it can be moved back to its original list such as "Errands" or "Work". Given how the app's main utility revolves around multitasking and navigating quickly around multiple lists, I feel this feature is totally in the app's ethos.

For those who are impatient, a debug APK is available here: https://github.com/lrq3000/OneList/releases/tag/storage-fix-and-more-features-bugfix5

Important note: this PR increases the minimum API from 16 to 19 to support the minimum libraries required for SimpleStorage to work, so by merging this we drop support for Android versions older than 19 (KITKAT).

@lolo-io This PR is fully functional, and if you don't like the additional features, you can just cherry-pick whichever commits you want, I rebased to separate them cleanly.

However there are a few minor things that could be improved:

• Currently, the Share and Share All functions are represented by 2 different buttons. They could be merged into one, with a dialog being created to allow user to select between "Share this list" or "Share all lists".
• In the Preferences, I added an option "storage_force" which, when clicked, copies all lists into the new default path. I implemented this to ease the transition of current users, so they can easily copy all their lists to the new storage. Although this option is not destructive, it would be better if it asked the user to confirm before being applied, to avoid misclicks.
• bug: can move new tasks to the very bottom of the list, then trying to move it will make the app crash. New tasks with option "Add to the bottom" should be created at the bottom but before done tasks!
• bug: when force storage update, path of lists are not updated on UI in EditListDialog until app is closed and reopened
• bug: force storage update not working when selecting App private storage, should just delete list.path
• portugues translations are missing for my additions, so it's in english until someone translate in this language. But the French translation is done. (won't do, I cannot translate in portuguese unfortunately, help is welcome!)

As you can see, these are very minor issues, it's just that I am not experienced enough with making dialogs. We only need very simple dialogs to make these two options perfectly fine. This can be done at a later point in time. This is now done.

Also note that I left Log.d calls on purpose, as they are anyway muted and skipped in the compiled app, but are very helpful if we ever need to debug storage issues again (especially if users need to debug! You can ask for a dump and it will tell you right away where there is an issue and if permissions were correctly granted).