QQuickTextInput: calculate height of cursor rect using QTextLine::height()

Calculating the height of the line using ascent + descent seems
inaccurate, since the result will not match what ends up being
drawn. QQuickTextEdit uses instead QTextLine::height() for the
same function, and this works correct.
Since there seems to be no reason to reinvent how to calculate the
height when the line already has a function for that, and since the
result also seems to be wrong, we change the implementation to
use QTextLine::height().

Change-Id: I9c9cd4360b6d4cfd3582756c4efdff9c02065789
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>