usecases: port to android

[FloVanGH]

• usescases demo can no be run on android
• the sidebar of the mail example is now responsive (can be open as popup on portrait mode)
• builtin osk is hidden on android

I want to make use of material style for the usecase demo on android in an upcoming PR.

[ogoffart]

Nice work!
I tried it on android and it kind of works, but there is some issues:

• I'd say it should use the material style on android (we should change the build.rs to use the material style depending on the TARGET_OS, instead of cosmic)
• The hamburger menu only applies to the "mail" tab, but the side bar comes over the tab. But I guess that's OK.
• Trying to fill the reply text edit doesn't work. the keyboard disapear as soon as you start typing. Probably a bug in Slint.

Overall, I think this is a good design. So this should be merged for now.
Too bad that it still has bug or exposes Slint bugs

[FloVanGH]

Nice work! I tried it on android and it kind of works, but there is some issues:

* I'd say it should use the material style on android (we should change the build.rs to use the material style depending on the TARGET_OS, instead of cosmic)

* The hamburger menu only applies to the "mail" tab, but the side bar comes over the tab. But I guess that's OK.

* Trying to fill the reply text edit doesn't work. the keyboard disapear as soon as you start typing. Probably a bug in Slint.

Overall, I think this is a good design. So this should be merged for now. Too bad that it still has bug or exposes Slint bugs

Thabk you. My plan is to do the material style in the next pr. You're right. The menu should only visible on the mail view.