2nd attempt to fix the bug described in commit 00221089 ("Qt5
IM allow committing empty strings") and various siblings of it.
This also reverts it.

What I see is calls with "all-empty" events (preedit, commit and
replacementLength() == 0; no QInputMethodEvent::Attribute), some
from QIBusPlatformInputContext::updatePreeditText.

There are various Writer document edit states with (selected)
text, undo, cursor position and focus changes to other windows via
Ctrl+Tab, which will result in inputMethodEvent calls totally in
contrast to the expected text state, all somehow always related to
all-empty events. They currently result in wrongly deleted
selected text, change of selection, cursor movement or general
change of text from old preedit. Most time on focus out / window
change, some times at first meta-key press after focus in.

This patch tries hard not to corrupt Writers edit state with these
all-empty events. No idea if this is some bug on LO's qt5 side or
expected, but KDE kate and VCL gtk3 and gen work fine, so I assume
Qt's behaviour is correct. FWIW gtk3 also does some extended IM
handling with focus, so probably this is the Qt equivalent of it.
But then I couldn't find some eqivalent code in Qt's source code.
I actually expected an even more complex solution (if this really
fixes all cases).

Works for a multitude of tests I tried to come up with, but is
quite probably not the final fix to this, as qt5 current doesn't
handle replacementStart() and replacementLength() at all.
Also never saw a call to Qt5Frame::EndExtTextInput.

Change-Id: I4210e0588041cfb4d80dbdfdb937e430a5f7cbfb
Reviewed-on: https://gerrit.libreoffice.org/71988
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>