add tooltip support

[mohsen77sk]

No description provided.

[PowerKiKi]

If you are trying to add tooltip, then you can already have standard tooltips on each button when mouse is hovering, without any modification to this lib.

On the other hand, if you are trying to solve #5, and add labels, then tooltip is probably not the right tool. As you can see in the video below tooltips will act as tooltip and appear/disappear on hover. That looks severly broken and is not what we expect from label.

blinking-tooltips.mp4

Also it does not match the material spec design:

I'll be closing this PR for now, but feel free to re-open a PR if you are willing to add support for labels, not tooltips, that follow the spec and that is covered by unit tests.