Adjust preprocessor guards for Qt5/X11.

Adjust preprocessor guards for Qt5/X11.

This fixes the -geometry command line option and restores the
"Use icons from system's theme" checkbox in the preferences.

There is still code addressing Qt4 and xlib that has to be
audited. This code cannot be compiled with Qt5 because the
default backend is now xcb and not xlib. I have marked such
code with a "FIXME QT5" comment.