All Gimp*ActionEntry (except GimpProcedureActionEntry) now have a short_label
member.
This commit doesn't add any new short label yet. It just fixes the struct usage,
and fixes a few localization contexts here and there when I saw such broken
strings.
I also fixed a few gradient editor action strings which were not proper labels
(like "splitmidpoint" or "splituniform", or missing uppercase, etc.).
These actions can be activated with a double value. These will be useful
to create new types of size action, which are based on accurate pixel
values instead of an enum hacked to set per-mille values between a
min/max.
