Bug 789764 - Please add Paste In Place feature

Add "In Place" variants for all sorts of pasting:

- extend the GimpPasteType enum with IN_PLACE values
- add the needed actions and menu items
- merge the action callbacks into one, taking an enum value as parameter
- refactor the pasting code in gimp-edit.c into smaller functions

We probably have too menu items in the "Edit" menu now, needs to be
sorted out.

menus/buffers-menu.xml

@@ -4,8 +4,11 @@
 <ui>
   <popup action="buffers-popup">
     <menuitem action="buffers-paste" />
+    <menuitem action="buffers-paste-in-place" />
     <menuitem action="buffers-paste-into" />
+    <menuitem action="buffers-paste-into-in-place" />
     <menuitem action="buffers-paste-as-new-layer" />
+    <menuitem action="buffers-paste-as-new-layer-in-place" />
     <menuitem action="buffers-paste-as-new-image" />
     <menuitem action="buffers-delete" />
     <separator />

menus/image-menu.xml.in

@@ -193,10 +193,13 @@
       </placeholder>
       <placeholder name="Paste">
         <menuitem action="edit-paste" />
+        <menuitem action="edit-paste-in-place" />
         <menuitem action="edit-paste-into" />
+        <menuitem action="edit-paste-into-in-place" />
       </placeholder>
       <menu action="edit-paste-as-menu" name="Paste as">
         <menuitem action="edit-paste-as-new-layer" />
+        <menuitem action="edit-paste-as-new-layer-in-place" />
         <menuitem action="edit-paste-as-new-image-short" />
       </menu>
       <menu action="edit-buffer-menu" name="Buffer">