cmake: fix installation of XDG mimetypes XML

Look for SharedMimeInfo and use the proper cmake macros of it to
install the XML for the XDG mimetypes to the correct place and in the
correct way.
cmake: remove installation of basket.desktop using xdg-desktop-menu

basket.desktop is already installed correctly from src/CMakeLists.txt,
so no need to do it again (and wrongly) here.
cmake: fix dependency handling of .po files

Create the "translations" target with the proper dependencies, so the
.po files are not rebuilt every time (but only when changed).
merge pull request #15 (dependency handling of .po)