iOS: replace custom `UITextInput` with a native `UITextView` (#1121)

* iOS: replace custom `UITextInput` with a native `UITextView`

`UITextView` is a full system-native keyboard client, so we get all the
major features for free: language HUD pill and the complete globe/Ctrl+Space
shortcut to cycle between IMEs/languages.

Makepad basically just mirrors the state of the system native text view,
via the `full_state_sync`, but the actual native text view is kept invisible
so it doesn't interfere with what we render in Makepad's TextInput.
Notably, the Full Keyboard Accessibility setting now does work properly,
whereas it did not before with our UITextInput-based approach.

We also make sure that arrow keys, nav keys, auto-repeat, and modifiers
are properly hanlded so we can retain the expected kbd shortcuts,
like other desktop platforms.

* iOS: remove the old `UITextInput` connection with the Makepad TextInput

We've now switched to the native UITextview, so we don't need this any more.

* iOS: fix desync during fast typing

Ensure there's no race between the native UITextView and
Makepad's TextInput, as the Enter/REturn key needs special handling
w.r.t. how `pressesBegan` gets it (From a real hardware kbd).

* TextInput: more iOS integration, and text input types

more native integration for things like username/password,
new password fields, email, address, URLs, etc.
These tell iOS to change the keyboard layout/type for the text input.

* iOS: don't let Full Keyboard Access focus on our hidden native cursor

* cleanup

* iOS/TextInput: fix perf issues

* iOS TextInput: more fixes for read-only efficiency, and filtered input

Also port some of these fixes to Android's IME integration layer

* iOS/TextInput: hide the native caret iOS draws during autocorrect

but still allow the "decline autocorrect" bubble to popup where that
hidden caret is located (and the CJK candidate window in the same spot)