Allow users to delete "user-perceived charaters" from text input widgets

Currently, pressing backspace in a text input widget (QLineEdit, QTextEdit, etc) results in a single code point being deleted. Whilst this is desirable in many situations, there are situations where the user may expect the entire "user-perceived character" to be deleted.

An example of this situation might be where the user has entered an emoji from an emoji palette, as found in many native mobile keyboards, but then wishes to delete the emoji. The user would not expect the emoji to be broken up into its combined code points. This situation is particularly likley to occur on mobile.

I have attached an example showing the current and desired behaviour, derived from a customer issue.

Unicode Standard Annex #29 also provides further details regarding Unicode text segmentation: http://www.unicode.org/reports/tr29/