Fix i18n of `iconText` in `.action` files
It looks like this never worked before. Most actions ended up with a translated iconText because there is a fallback in kis_action_registry.cpp that tries to translate the string without using a context at all. Coincidentally, "Invert Selection" has neither an icon and a matching entry for the iconText so it ends up displaying in English.
This MR fixes action_i18n.pl to extract iconText for translations. I also took the opportunity to fix some related strings issues in the .action files, and also improve the i18n comment slightly so translators can have a slightly better idea what the string is for without referencing the source .action file.
This adds 38 translatable strings. Here is the POT file I generated locally as a test. Note: this is slightly different than what Scripty will generate -- mine has duplicated comments for some reason.
/cc @tysontan