As hinted in d09a2a6f
We now use the word 'bundle' to signify "program files in the same prefix"
(e.g. .appimage, .zip, .app). This is in line with our source and dev-docs
(just take a search in the repo). So, appimage and windows scripts changed.
The word 'package' normally means "program files distributed for install in
the same prefix or not" (e.g. .deb, .msi, .dmg). This is in line with CMake
naming of some commands, but meson prefers to call 'dist', which we use more.
So, this partly reverts some things of GNOME/gimp!1171 and reinforce others
for even more "rationality" in the overall build structure of GIMP.
AppImage is pretty fast to make, like the win crossbuild; and portable,
being very appropriate to do quick tests on Linux when pushing to git.
The overall organization of Debian jobs was changed to take advantage
of this and make things less complicated (but less clear at first sight).
I reinforce that this was the most efficent way to make the AppImage.
