Add options for app appearance

[harshad1]

This PR Adds a new Appearance section in setting with a few new options:

1. Set the top and bottom bars to match background (default is Markor blue). This gives the app a more modern flat appearance.
2. Show / hide the Main activity tab titles (default is show) . This gives us back some vertical space
3. Show / hide the File browser divider lines. For a flat appearance

I have also moved the Dark / Light theme option in here

Default appearance is unchanged.

In these images you can see

1. The new appearance section (2nd from the top)
2. The options under this section
3. MainActivity showing: Flat navigation and tab bar colors, no tab titles, no file dividers