Implement deselect() function for TextInput and TextEdit elements

QML is style independent technology and doesn't come with ready-made finger-usable text selection controls. Luckily those are easy to implement using MouseArea, TextEdit and TextInput functions select(), selectWord and selectAll, and by creating your own selection handles using QML.

Clearing active selection is something all text selection controls need to implement. Currently to do that from QML you basically call "select(cursorPosition, cursorPosition)" with the parameters start and end position having the same value. For a convenience a new function deselect(), already available for C++ editors like QLineEdit, should be introduced.