many change for deepin apps

1. add media-plugins/python-mmkeys:
    deepin-music-player need python-mmkeys to enable global hotkey, and mmkeys.so is part of sonata. Actually, we do not need to install the whole sonata, so I create this ebuild to compile and install just mmkeys.so. 
    To solve dependency conflict with sonata, I write “!media-sound/sonata” in DEPEND variable. Is that correct?

2. add metadata.xml files for deepin apps 

3. add command "python2" to deepin-music-player, deepin-media-player and deepin-screenshot, so that we do not need to eselect python to version 2.7