Signal: assetAboutToBeExported

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**Is your feature request related to a problem? Please describe.**
I've got an extension that listens to assetAboutToBeSaved to do last-minute corrections. Examples:
- ensuring that polygon vertices are in counter-clockwise winding order
- ensuring that vertices are snapped to the nearest pixel coordinate
- apply rotation and offset

This works well, but the signal is not triggered when exporting. Thus, when changes are made since the last save and a user then hits export, an invalid file can be generated

**Describe the solution you'd like**
A new signal `assetAboutToBeExported` that works the same way as `assetAboutToBeSaved`. For consistency perhaps `assetExported` as well

**Describe alternatives you've considered**
- An `objectEdited` signal emitted whenever anything about an object changes. But there are usecases for layers, tilesets etc. too.
- Doing the corrections to the map in the `write` function of a map format. Not allowed; the map is read-only.
- Completely parsing the map and writing it to a custom format, doing checks and fixes along the way. Lots of work, plus the result is not visible to the user since it is only applied to the exported file; not the map
- Doing the checks in the `write` function of a map format and returning an error message "please save before exporting" when issues are found. That seems a bit annoying when the issues could be fixed automatically.

Start by locating the existing assetAboutToBeSaved signal and tracing the export path, including where map-format write functions are called. Compare the available signal timing with the requested assetAboutToBeExported and optional assetExported behavior; done means extensions can apply last-minute corrections before export without modifying the read-only map.

Written by the indexing model from the issue text.