Revert "app: action search should search accross all available actions."

This reverts commit 53b3673b.

Had to revert these two commits, quoting comment 6 of the bug:

Thinking again, that entire change is unfortunately wrong, the only
right UI manager is in fact

gimp_ui_managers_from_name ("<Image>")->data

because it's the global popup <Image> UI manager which is independent
of a display and it always in the right state for the currently
active image, all other UI managers are wrong.