[WIP] [build wheel] Introduce TextInput provider (Android, iOS and "Legacy" Window provider)

[misl6]

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• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
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A little brief

This is definitely not intended to be merged as-is, instead is just a POC for supporting additional textinput methods. (The idea is to treat the textinput provider as other core providers we have in Kivy (Camera, Window, ...))

It works (but sometimes it crashes, as I needed to achieve a result, and I did not took care about everything 😀) on both iOS and Android with platform-specific providers, and offers the same support as before with window-provider events on other platforms.

POC.mp4

Why this is needed?

• The window provider (SDL in that case) does not have (and is not required to?) all the events and methods to access the complete feature set that the (mostly mobile) platforms offer.
• We currently only support basic text input methods (we basically receive keystrokes from the keyboard and change the TextInput widget text property accordingly). This may be fine for languages that do not require an Input Method Editor (IME) to input text.
• The window provider (SDL in that case) is not able to handle multiple TextInput fields at the same time. That breaks the keyboard suggestions for us.
• The current implementation does not allow us to handle autofills (E.g. for password managers)
• The current implementation does not allow us to handle accessibility features on textinputs.

Some related issues:

• Keyboard suggestions on iOS may be buggy #8172
• Kivy Text Input Field not accepting all android keyboards (like handwriting, regional language ) and not displaying multi language Unicode text #7741
• IME input left arrow key causing incorrect insertion #6898
• TextInput limited to 1 character when Swype with word suggestion turned on #2694
• kivy on ios and android keyborad IME #5739
• Android 6.0.1 TextInput: Spacebar nor swipe nor autocomplete works? #4605
• When typing Sinhala language from Gboard (Google Keyboard), English language is typed instead of Sinhala in TextInput. #8185
• Text Input typing bug #7415
• Android - Textinput can't type Unicode. #7294
• Keyboard won't save input language preference #7184
• Keyboard not same as in all other apps in android #7053
• Backspace doesn't work correctly if input_type is set to "number" #8232
• Backspace key on android not clearing a text that is pasted #7375
• Backspace in TextInput on android does not work with on_text_validate method #8267
• Keyboard suggestions does not work in Microsoft SwiftKey #8272
• possibly many other issues 😭

How we tried to fix this issue in past:

• Disabling the keyboard suggestions (spoiler: does not work for all the soft keyboards)
• Using textinput events from SDL (spoiler: these events are not enough to handle everything)

TODO:

• Re-think FocusBehavior, possibly without breaking the current API. The keyboard needs (and should) be handled by the textinput provider.
• ...