Improve post_per_page UX

[iansvo]

Fixes #1527
Related: https://github.com/tomusborne/generateblocks-pro/pull/769

[tomusborne]

@iansvo I'm still able to save super negative numbers in the posts per page field, is that intended?

I also was never able to type in non-numeric values, even in the current release branch.

[iansvo]

Are you able to enter in a non-number for a post query or a pro query?

The code here should force that input to be a number with a min of -1, so seems like maybe you're looking at pro. If so you need the related PR.

[tomusborne]

@iansvo I'm not able to add a non-numeric value in the release branch, or this branch.

I'm still able to add a value like -12345 in this branch.

This is happening with GB Pro deactivated, and using a regular post query.

[iansvo]

Ok so apparently we need to handle the minimum in the onChange as well as in the attribute. I also saw that the Control was not quite working correctly with this when I did the onChange fix, which I assume is because the componentValue becomes unsynced with the value prop.

This is now fixed in my latest commit.