app, menus: move "GEGL Operation" from Tools to Filters > Generic menu.
See the discussion in #5339. Basically this is now technically a tool (i.e. a child class of GimpTool) and tools can be activated anytime, even when no images are opened, i.e. when they are useless (for instance paint tools). Filters on the other hand were historically only activatable with opened images. With time, tools and filters difference got slimmer (until there are technically none nowadays where GEGL ops, levels or curves are implemented as GimpTool too) if not only a conceptual difference. Since here GEGL Operation is really more on the "filter" side, let's just move it to the "Filters > Generic" submenu, just next to "GEGL Graph", also removing the mention of it being a "tool" from the help message. Then I will merge !402 for it to be (in)active depending on images, as other filters do.