Allow specifying arbitrary adwaita versions
Issue written based on the discussion in #86 

Currently owlkettle requires you to add the flag `-d:adwaita12` if you wish to compile for environments where you know the adwaita lib is installed with a version of 1.2 or higher.

This is done for widgets that were added in version 1.2 of the adwaita lib.

This doesn't scale well if later down the road we have widgets that were added in 1.4, introducing a new `-d:adwaita14` flag and now you need to specify both `-d:adwaita12` and `-d:adwaita14` if you need both their widgets.

Thus it makes sense to rather move over to compilation flags such as `-d:adwMajor=1 -d=adwMinor=2` where you define a specific version and based on that sections of code are either compiled in or out.