[2.3] [Re-build] Manager Theme Windows Fixes

This PR fixes most importantly a bug that was introduced in #11689, which threw an error when trying to drop an element with properties into a resource content field (treedrop), the problem was, that this special window creates a loading mask, on which an unsupported method (addClass()) was called by the JS that handles the classes for the CSS3 transitions when the windows show.

Furthermore I normalized all windows to either 400px width, 600px width and for the resource quick update 700 (was already like that) and tested all of them again.

These changes also prevent windows from being oversized by autoHeight: true on smaller viewports (like my macbook air 11"), good example was the plugin quick update window.

Some small CSS adjustments for scrollbars were made too.
Would it be cleaner to just get rid of the line entirely if it's commented out? We do have git for history..