Prevent excessive calls to deleteBackward method which may cause crashes on Android

[rh101]

Describe your changes

The changes in this PR will prevent situations where a call to TextField::deleteBackward is made for every single character when deleting text in large blocks, such as via a Select All -> Backspace operation using the Android virtual keyboard. When many consecutive calls to deleteBackward are made in a single update/frame, it seems to result in a crash.

Just note that this does not change the behavior when the user is deleting one character at a time, since it will still call TextField::deleteBackward once per character. The test cases in cpp-tests, 49:Node: Text Input confirm that the behaviour is still the same.

Issue ticket number and link

This possibly relates to all the following issues:
#2240
#2241
#2242

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