Add support to change theme from settings dialog
Changes:
- The theme setting entry was added to the General tab since we currently don't have a bunch of settings yet, it can be rearranged later if preferred.
- MarknoteSettings becomes a singleton so it can be easier to access from C++ side.
- Config becomes ConfigHelper at QML side since it now only provides the information of
fontFamilies
. - the reset setting button was removed since it can actually only reset the font size setting.