File > Export menu: add new commands, overhaul existing structure

[tannerhelland]

The File > Export menu has been reworked. It now provides:

• Export image to file (supports all the same features as "save as", but does not modify image save state)
• Export layers to file (new feature, allows the user to export all layers / all visible layers / only current layer to individual image file(s))
• Export animation (replaces the old individual "export to animated GIF, PNG, JXL, WebP" menus).

This fixes #513 and relates to #467 (and possibly others).

Apologies that it took me some time to get this implemented!