Allow setting item label via string resource

[Tunous]

This pull request adds a way to set item labels by using string resources instead of regular strings.

Example:

val popupMenu = popupMenu {
    section {
        item {
            labelRes = R.string.copy
        }
    }
}

[zawadz88]

Hi @Tunous,
Thanks for dropping this PR (and the other one)!
I'll review it shortly :)

[zawadz88]

Just this one change and this can merged :)
I'll review your other PRs and once they're merged I'll prepare a new release.
Thanks for contributing!

[zawadz88]

This is used in the sample app only so it shouldn't be in the library.
Could you move it to sample/src/main/res/values/strings.xml instead, please?

[Tunous]

Done.

[Tunous]

I'll review your other PRs and once they're merged I'll prepare a new release.

Cool, but you don't have to be too quick with that. I think I'll want to make at least one more pull request. (For customizable max height/width, unless that's already possible. Not sure if there'll be anything else.)