Add version macros

Version macros provide a way to find out version information at compile
time.
Remove generated files from includes
Add include guards for playerctl.h

Only <playerctl/playerctl.h> can be included directly.

Additionally include playerctl.h from the include path instead of the
current directory.
Add playerctl-version.h to ignore and cleanfiles
Bugfix: initialize bus_name to NULL in getter

This memory error may cause the player to not throw an error when no bus
name can be found.