Split the LastFm plugin into multiple files and make it work again

I'm wondering why I originally put everything into one file...:)
Shuffled some code in Lyrics plugin a bit
Switch from plugin .desktop files to Qt5 plugin JSON metadata system

This breaks loading of plugins, as PluginsManager has not yet been ported
to look for plugins binaries instead of .desktop files. It just ports the
plugins to use the JSON metadata files
Implement loading Qt5 plugin metadata
Remove -fno-check-new CXX flag

This flag is not supported by clang and generates lots of warnings at
build time.
Fix 'unused parameters' warnings

We intentionally ignore the Qxt warnings.
Clean up TODO
Clean up translations mess in CMakeLists