Improve the CMake option stuff

We now use cmake_dependent_option() to express dependencies between
the various options that you can give to CMake for configuring Quassel;
for example, enabling KDE4 now forces some options to OFF regardless
of the user-selected value (which will, however, be preserved in case
KDE integration is disabled again later).

This also renames WITH_QT5 to USE_QT5 (and adds a USE_QT4, too); this seems
to have emerged as the standard way to select a Qt version for dual-Qt
packages. However, Qt4 is still selected by default unless you force Qt5
by setting the USE_QT5 flag. BTW, don't get your hopes up, Qt5 support is
still broken... Once it's working, we may select Qt5 as default for certain
platforms.

Additionally we renamed QT to QT_PATH to make the function of this flag
more clear.