Fix-up the Export dialogs

[hgonomeg]

The "Export Map" and "Export Map Fragment" dialogs are a mess. They should be overlays in the style of the "Get Monomer" overlay.

(The resulting file choosers should of course have their own dialog - like any open file or save file operation)

Tasks

• [x] Convert the dialogs into overlays
• [x] Find out how they should work in the first place
• [x] Fix their usage