Simplify props API of ExportMenu

[axelboc]

Preparation for #1063

The getFormatURL prop was confusing. It made it look like the URLs were generated lazily, which isn't the case -- they're generated during render. Also, we used to end up with two arrays, formats and urls, so we had to access one by index in the map loop.

[loichuder]

Nice

You could filter exportEntries to remove entries with url = undefined before passing it to ExportMenu. This would remove some checks in ExportMenu and avoid passing entries that are not relevant to the ExportMenu.

[axelboc]

I had this at first, but it meant that the toolbars had to do the filtering, which felt like a bit too much unnecessarily duplication. It may evolve again, so I went for what felt like the simplest solution first.

[loichuder]

I had this at first, but it meant that the toolbars had to do the filtering, which felt like a bit too much unnecessarily duplication. It may evolve again, so I went for what felt like the simplest solution first.

Sure, no qualms about merging this as it is