app, menus, plug-ins: move some advanced paste actions into submenu.

There are so many paste options and I feel that the "Paste into Selection*"
actions are advanved enough that they can go into submenus. So I move them into
"Paste as". The other reason is that I'm going to add 2 more options!

I hesitated to rename the "Paste as" submenu but we couldn't find a good naming
(except just "Paste" but it's redundant with the default action and "Paste…" but
this usually implies a dialog, not a submenu).

Last but not least, I renamed the various paste actions to contain the word
"Paste" in it. E.g. s/New Image/Paste as New Image/. The old naming made sense
when action labels were only displayed in menus. But nowadays they can be shown
in other more independant context, such as the action search (and just
displaying "New Image" in there is very misleading).