cmake: Consolidate compile settings

cmake: Consolidate compile settings

Consolidate the various places where we were setting compiler options
in QuasselCompileSettings.cmake. No longer include KDE's settings;
instead, take some inspiration from their setup and incorporate this
in our own, so we not only benefit when building against KDE
Frameworks.

Properly handle CMAKE_CXX_FLAGS and friends; append the flags to the
end of our own list of flags, so one can override what we define.
Clear the variables afterwards to avoid duplication.

Get rid of some MinGW options that have been around for a decade
and are most likely no longer required.