The "Fill" message can be either a noun or a verb. This commit
disambiguates the verb usages for translation through various
translation contexts.
The more involved change is in the generation of keymaps from paint
modes. By default, the enums defining brush names are in the default
context, but this commit changes the ones including a "Fill" item to
"Brush". In order to get the same contexts in the keymap, we introduce
a specific function in `paint.cc` to return the appropriate context
depending on the tool.
Issue reported by Gabriel Gazzán (@GabrielGazzan) in #43295.
