Theme manager

The theme manager will be able to adjust the internal theme of LaunchMenu.

This theme is already present within the code, and is consistently used in both LaunchMenu itself and in applets. It contains the following properties:

• Colors
• Elevations (shadows)
• Fonts
• Radii (rounding of boxes)
• Borders (box outlines)
• Spacings (distance around and between boxes)
• Icons

Currently LaunchMenu contains no way to customize these values. This theme manager applet should be able to change most, if not all, of these theme properties.

This would preferably be done by means of regular settings, in order to benefit of any features that settings may get in future such as exportability.