Issue #4352: Text copy & paste without format

Closes: #4352

Description

This merge request adds a Paste Unformatted Text item inside the right-click menu of Text Tool, which allows user to paste previously copied, possibly formatted (marked) text as plain text with no markups.

Behavior

20240306-0339-12.4548451

Current Problems

• Unsuccessful attempt to add shortcut (accelerator) to the Paste Unformatted Text item.

  Tried to define the corresponding accelerator inside text_tool_actions (app/actions/text-tool-actions.c:65) as { "<primary><shift><alt>V", NULL } (the same as in LibreOffice), but no actions were performed when the key combination was pressed. Modification of app/actions/edit-actions.c is possibly needed, but it would affect GIMP's overall actions at a larger scale. By adding correct key combination into the gimp_text_tool_editor_key_press function it now works.

• Decide which shortcut to use (Ctrl+Shift+Alt+V / Ctrl+Shift+V)

  LibreOffice uses Ctrl+Shift+Alt+V to paste unformatted text, but having to press four keys make it harder to use. Ctrl+Shift+V has its own usage in GIMP (creating new image from clipboard) but it still makes sense if it's only limited in the scope of text tool.