Implemented GuiHandler::hideAllWindows().

Also added some doxygen to better distinguishe between hideAllWindows() and toggleGui().
Added SettingsWindow stub implementation.
Integrated SettingsWindow into existing gui.

- GuiHandler creates SettingsWindow instance.
- GuiHandler has method showSettingsWindow() to show SettingsWindow instance.
- MainWindow connected settings button to GuiHandler::showSettingsWindow().
GuiHandler::hideAllWindows() not hiding MainWindow any more.
Added some functionality to SettingsWindow.
Merge branch 'dev' into settings
Using C++11 for settings branch.
Added audiosettings (stub only).
Added videosettings (stub only).
Created a sample serrings conf file.
Added Settings (stub only).
Added some basic attributes to VideoSettings.
Set up settings loading process (stub only).
Added XMLHelper class.
TileSetProcessor now derived from XMLHelper.
Now using tinyxml2::XMLError for error passing
Added some convenient error printing methods (XMLHelper).
Implemented VideoSettings logic.
Settings is now a singleton.
Fixed spelling mistake in VideoSettings.