Fixed settings.xml doctype.
Implemented VideoSettings::toTinyxml2Element().
Implemented Settings::saveToFile().
Some code cleanup in settingswindow.cpp.
Added default settings path to Settings.

This is the default path to the settings.xml file. You can now call Settings::loadFromFile() and Settings::saveToFile() without parameters which saves the settings to the default path.
SettingsWindow now able to read and set values from Settings.
Set up possibility to save to Settings in SettingsWindow (stub only).
SettingsWindow now saving tilesetpath to Settings.
Now able to toggle fullscreen in SettingsWindow.

Note: main.cpp not yet capable of switching to fullscreen.
Added fullscreen switch in main.cpp.