grub2: add Resolution property
grub2: add config.go
grub2: update GetAvailableResolutions()
grub2: refactor code, about writeSettings()
grub2: refactor code, save config file when set setttings
grub2: update fixSettings(), read config file
grub2: add properties FixSettingsAlways and EnableTheme
grub2: fix issues when running without dbus