Fix calculation of text margin if line edit contains side widgets

The previous implementation leads to infinite chain of showing/hidden
line edit under circumstances described in QTBUG-54676. We basically got
the situation when size hint were calculated differently depending on
the line edit visibility state. In this case toolbar layout have to
show/hide extension button and line edit a lot of times and can never
leave this "loop" (please note, that the chain is much more complicated
in reality):
Resize toolbar -> Set layout geometry -> Size is OK to display line edit
-> Set layout geometry -> Hide extension button -> Set layout geometry
(wrong size is calculated here, so "run out of space") -> Hide line edit
-> Set layout geometry -> Show extension button -> Set layout geometry -
> Size is OK to display line edit ... And we're in the "loop"

Clear button is hidden if there is no text in a line edit.
In the previous implementation, the button was always visible, only
opacity was changing in order to "hide" the button. It resulted to
incorrect size hints (regular and minimum).
In the current implementation the button is really hidden/shown, and
size hints calculated correctly.
Also updated unit test for line edit.

Remove code duplication in functions for calculation text margin

Fixes: QTBUG-54676
Change-Id: I4549c9ea98e10b750ba855a07037f6392276358b
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

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