[macOS] Put FlutterTextInputPlugin in view hierarchy

[knopp]

Before	After

Emoji picker is different than the other IME popovers. In order for the picker to be attached properly, FlutterTextInputPlugin must be in view hierarchy and first responder.

Notable changes in this PR:

• FlutterTextInputPlugin is added to hierarchy in TextInput.show, removed in TextInput.hide.
• Manual [TextInputContext activate] and deactivate calls are not needed anymore and were removed. The context gets activated automatically now. I also removed the TestTextInputContextIsKeptAlive unit test. Unfortunately I wasn't able to get the context to activate in "headless" unit test environment, but in actual application the context gets activated/deactivated as expected (tested by overriding the activate/deactivate methods on context).
• I've removed key up event synthesising in performKeyEquivalent:. The keyUp event monitor in FlutterViewController takes care of this. It needed to be modified to allow TextInputPlugin as first responder (which is also works with accessibility text field)
• [FlutterTestInputPlugin performKeyEquivalent:] now simply calls [FlutterViewController keyDown:]. With one caveat: When the CMD+CTRL+SPACE emoji picker shortcut event gets redispached to nextResponder by FlutterKeyboardManager, the nextResponder is usually NSWindow and it routes the event to [FlutterTestInputPlugin performKeyEquivalent:]. Sending it again to controller would result in endless cycle, so the FlutterTextInputPlugin checks if the event in question is currently being redispatched by manager and if it is, it will not handle it. Not handling the event is also required for the emoji picker to actually show up.
• I've replaced calls to becomeFirstResponder with [NSWindow makeFirstResponder:]. Unlike UIKit, in AppKit the becomeFirstResponder documentation explicitely says not to call this method directly and in some cases it leads to crash with unhandled NSInternalInconsistencyException.
• [FlutterTextField becomeFirstResponder], which was called directly has been replaced by [FlutterTextField startEditing]. This method sets FlutterTextPlugin as field editor to the FlutterTextField. This was previously happening as a side effect of becomeFirstResponder calling selectText.

Partially fixes flutter/flutter#85328

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[LongCatIsLooong]

/cc @cbracken

[LongCatIsLooong]

I couldn't find the place where the text input plugin is added to the view hierarchy. You mean with this change the accessibility text field still causes the text input plugin to be removed as a subview when it has the focus?

[knopp]

When FlutterTextField has focus and FlutterTextInputPlugin becomes the field editor of ([FlutterTextField startEditing]) the input plugin gets reparented inside the text field.

[LongCatIsLooong]

thanks for fixing this! 👍

[knopp]

This was the wrong fix. The TextInputField is actually not meant to be first responder. Instead the FlutterTextInputPlugin is supposed to be the first responder. What the original becomeFirstResponder did was not making the text field first responder, it made FlutterTextInputPlugin current field editor for the text field. It happened as a sideeffect of becomeFirstResponder calling selectText, which sets the field editor as current editor.

So instead of overriding becomeFirstResponder and calling it directly (which shouldn't be done) I replaced it with [FlutterTextField startEditing], which makes the text input plugin current field editor.

[dkwingsmt]

LGTM

[knopp]

@cbracken, @LongCatIsLooong, any objections to getting this merged?