Pinnable windows

Fix https://bugs.openmw.org/issues/530 and a little thing spotted during my tests.  

Speaking of the issue: I added a function 'getSkinWidget' in OEngine::GUI::Layout for consistency. This function is similar to the existing 'getWidget' but allow to access different GUI widgets (those defined in the skin instead of those defined in the layout).
Is it okay to modify OpenEngine or is it preferable to move this in MWGui?

Now about the other fix: the first commit changes the display conditions of pinned windows, which were displayed when the menu is shown and during dialogue and trading. With this commit, pinned windows are displayed during game, in console mode and (of course) in inventory mode. This differs from vanilla, as they were also displayed behind the rest window and the "quick keys" window. These two windows were modals. Since interacting with pinned windows was impossible, I think hiding them is functionally equivalent.