UI theme settings

[MikeSullivan7]

This PR impliments RasCAL being displayed in Light or Dark mode depending on the users system settings. As default, this will be synchronised with the OS settings, but it is possible to override this be explicitly selecting Light or Dark mode in the Settings Dialog.

Things to test:

• Open RasCAL and check that it's appearance aligns with your OS Settings.
• Close RasCAL, change your OS appearence settings, reopen RasCAL and check that it opens in the correct mode.
• Open a Project and go to settings, change to opposite theme (e.g. Light -> Dark) and accept changes.
• Reopen RasCAL and check it has remembered the most recent theme change.
• Check that clicking "Reset to Defaults" in Settings changes the theme back to "system" mode.