hotkeys/gtkui: add "skip to ..." entries

This commit adds "Skip to ..." entries to the playback menu and hotkeys list. "Skip to next/prev album/genre"/composer should be self-explanatory and (should) behave just like next and previous (track) do. "Skip to next/prev artist" takes a bounch of tag fields into account (%band%, %album artist%, %albumartist%, %artist% (in that order)). 

The hugh amount of added lines is mainly because of some code duplication - most of that feature is added twice, once in the hotkeys plugin and once in the gtkui plugin. 

Since I couldn´t get the boundled glade version to compile I added those parts manually - hope that´s ok.

Looking forward to some feedback.