TiGLCreator: Need to specify file ending when saving a screenshot.

[joergbrech]

If we don't explicitly add the file ending (e.g. .png) in the filepath, we get a not-very informative error message.

ERR 03/25 10:53:31 TIGLCreatorWidget.cpp:1275] Unable to save screenshot to file '/home/threepwood_guybrush/Screenshots/awesome_airplane'

At the very least, we should have a more informative error message. Even better, it would be nice if an appropriate file ending was added automatically, if it is missing.

Thanks @HannahGedler for the find.