Fix the findbox regression, part 2.

This gets rid of some race conditions in the find operation, by ensuring
that edit mode is only updated *after* mapping a subpatch window.

This bug manifested itself when searching for objects in subpatches
which are not visible at the time the find operation gets invoked. In
this case, on the GUI side we both have to create the subpatch window
and put it in edit mode. In the previous implementation these two
operations would be executed separately and asynchronously. The end
result was that usually the latter operation setting the edit mode in
the GUI would win out, and the edit mode would later get overwritten
again when the window was created.

We fixed that race condition by ensuring that the engine is queried for
its current edit mode status after creating a canvas window. The engine
replies with a call to gui_canvas_set_editmode() in the GUI and thus the
edit mode in the GUI now properly reflects the status in the engine
after a window is mapped.

TL;DR: This fix should now make sure that edit mode on the GUI side
reflects that of the engine at all times.