support swapping left/right panels in darkroom

[dterrahe]

• new preference under darkroom/modules

Other changes/refinements:

• add an event box in the empty space at the bottom of the panels and connect tooltip and drag/drop handler there. Not only does this simplify things, it prevents tooltips shining through in the wrong places, like resizable listviews.
  
• To avoid this empty event box from getting destroyed, clean up destruction of iops. It seems that the error conditions that existing in the past that made it "safer" to hide instead of destroy widgets no longer exist. Obviously, needs (more) testing (adding, then removing, then dragging iops (to top and bottom of list etc)
• further simplify creation of side panels. There was some leftover stuff from when they still could sometimes show horizontal scrollbars. As usual with any cleanup: they can break stuff, so test scroll behavior etc.
• scroll the currently dragged module or the one above or below it into view, even if the dragged module cannot be dropped there. This helps moving a module that is currently shown at the top upwards or one that is at the bottom downward.
• got rid of a few GTK-CRITICALS because sometimes gtk can't keep up allocating widgets or starts a d&d without a source.
• also made sure to clean up drop zones at the end in case drag/leave messages got delivered out of order in a hurry to process a drop.

Maybe fixes #17478

[TurboGit]

A simple comment for now. Works fine otherwise.

[TurboGit]

Works for me, thanks!