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)
* Add qt5 build support to lxqt-panel and lxqt-config