Refactor: Move generic widgets to their own place and make them useful for reporting errors to the user

The rather generic mdcDialog function was moved to MagicFolders.hs a while ago.

This is not a good place for that code.

It displays a modal dialog, which can be useful in many situations.

@shae and I would like to use it for reporting errors to the user.

TODO: Refactor mdcDialog(), errorBox() and friends out of MagicFolders.hs and into their own module so they can be re-used.