Add TextEditor core provider, rewrite TextInput UI element on top of it

[misl6]

Similar to #8574, handling text input internationally is a painful task, and sometimes we do not even have a proper API to handle it, as text input is something strictly tied with the OS-backed TextEdit widget. (This is not just Kivy's case, but also other frameworks are on the same boat).

The current status:

• The window provider (SDL in most cases) 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 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.

We have a POC #8226, whose major intention was to test the feasibility of using OS-backed hidden widgets to get the events. That worked quite fine. We need to find out how to manage it for Desktop platforms, and write a unified API + keep the support for the legacy (keystrokes) version.

Please note that this issue is partially on hold until #8574 is assessed, since kivy.ui.textinput will use TextLayout core provider (for layouting, rendering, getting selection rectangles, getting glyph at cursor pos, ...) and TextEditor core provider for handling the textedit events.

Related issues to consider:

• Feature Request: right to left support for TextInput #1619
• Cursor navigation keys are very incomplete #1863
• TextInput crashes while using clipboard bubble #1945
• Textinput: past/copy problems #2418
• TextInput limited to 1 character when Swype with word suggestion turned on #2694
• How to trigger Windows 8.1 on screen keyboard? #2998
• Windows 8.1 handwriting recognition via OS keyboard #2999
• TextInput focus changes and native keyboard #3184
• "readonly" textinput still opens a keyboard in "multi" mode. #3612
• Can't type special characters into TextInput #3459
• Chinese Input Broken on iOS device #4079
• TextInput large string rendering issues #4166
• TextInput : New feature on input_type #4420
• Android 6.0.1 TextInput: Spacebar nor swipe nor autocomplete works? #4605
• window_sdl2.py: TextInput crashes on numeric input in Android #5043
• TextInput on android can not wake up virtual keyboard #5550
• TextInput insert_text and input_filter always destroy active selection #5938
• the delete key behavior error when using the Chinese input method (输中文过程中delete键会删掉文本中的内容) #6532
• TextInput().do_cursor_movement() doesn't work #6783
• Missing characters in TextInput field from fast sources (e.g. barcode scanners) #6801
• IME input left arrow key causing incorrect insertion #6898
• Kivy App Textinput Keyboard not open #7035
• Keyboard not same as in all other apps in android #7053
• Keyboard won't save input language preference #7184
• One-step undo in TextInput #7237
• Android - Textinput can't type Unicode. #7294
• Text Input typing bug #7415
• TextInput multiline wrap/cursor issues #7474
• On X11, dead keys can only be differentiated by scancode #7506
• TextInput Bug ( a -> l , 1 -> 9 , q -> o , z -> . etc ) #7627
• keyboard issue when switching between TextInput widgets on android #7698
• kivy TextInput does not work with windows clipboard #7699
• Kivy Text Input Field not accepting all android keyboards (like handwriting, regional language ) and not displaying multi language Unicode text #7741
• TextInput truncating to 32 chars when using Speech-to-text #8061
• When typing Sinhala language from Gboard (Google Keyboard), English language is typed instead of Sinhala in TextInput. #8185
• Backspace doesn't work correctly if input_type is set to "number" #8232
• Backspace in TextInput on android does not work with on_text_validate method #8267
• Support international languages #8444
• multiline TextInput text selection on android #8474
• TextInput does not support "home" and "end" commands on windows #8475
• issue with textinput keyboard_suggestions or InputType on android when using backspace #8625

[Julian-O]

textinput.py stands at 4,050 lines of code. TextInput is a single class with 3,600 lines of code.

This would be an excellent opportunity to break it down into much, much smaller parts with subclassing, mixins, delegation, etc. and improve unit-test coverage.