no need to manually set the display resolution anymore
CDisplaySettings: improve screen options filler to determine the current screen
settings: make sure videoscreen.screen/resolution change if videoscreen.screenmode is changed
settings: add OnSettingPropertyChanged callback to ISettingCallback
settings: add IsEnabled() to CSetting
settings: move dependency logic from CGUIWindowSettingsCategory to CSettingsManager
CDisplaySettings: don't show a prompt when switching from/to windowed mode
remove unused methods from CSettings