[0.11.22] Demo scenario cannot accomplished if GUI is open

[hron84]

I ran into a bit rare scenario: while the radar quest finished I had an open GUI window (in this case the boiler's GUI) and while I put a coal into the boiler, the radar finished the scanning of the last section. Because this I were not able to click on finishing scenario since the game did not let me close the boiler GUI.

Proposed solution: if scenario finishing, automagically close all open GUIs before the summary dialog pops up.

[slpwnd]

When the demo is finished the game should be stopped (gray overlay) and on top of that the ending screen gui appears. I tested it with a game gui opened (I didn't wait for the level to finish - instead altered the script a bit=)) and it works just fine (0.12 branch).