Added exceptions for popover appearance.
Making sure we have a valid window reference when setting the default tint color.
Removed custom popover appearance.
Cleaning up comments.

`tintColor` is also not an appearance selector, but we have been relaying on it for some time now, so let's not point that out just for `barStyle`.
Implemented and alternative approach to appearance customization.
Reverted a deluging setting.
Merge pull request #48 from PSPDFKit/matej/appearance-fixes

[READY] Appearance fixes