GIMP 3 / GTK3 migration

[bbbbbr]

The migration to GTK3 will likely break the plugin. Deprecated GTK and libgimp function calls will need to be replaced.

No action will be taken until after the GIMP 3 release. This is just a placeholder for now.

Also see:

• Incompatibility with GTK 3.0 bootchk/resynthesizer#59
• https://wiki.gimp.org/wiki/Roadmap#GIMP_3.0

[zvezdochiot]

Resize functions are independent of gtk and gimp in general.
The code should be divided into two directories: src/gtk2/src/gtk3 and src/lib.
Change Makefile: first compile the static library libpas.a and link it to gui already.
This will allow you to select the "face" when compiling.

ℹ️ See #9