Fix LXQT-9999 Build Failures

* lxqt now requires qtmimetypes library to compile
* New Ebuild for qtmimetypes
* Patch for build failures in lxqt-config
* Add dependency qtmimetypes use flag in libqtxdg in order to build
  liblxqt and lxqt-panel packages
  Note: razor-qt does not require the qtmimetypes use flag (yet)
Not sure what you mean by "make USE qtmimetypes be available only if qt4 USE is set". If you want a REQUIRED_USE constraint, I'm against it in this case. Just silently ignore the flag when building against qt5.