Proposing a new URL type: downloadpage

[probonopd]

I am proposing a new URL type: downloadpage.

Rationale:
Sites like https://appimage.github.io/ may want to link to a project's download page, not just the homepage. Currently there is no way to do that using the AppStream metainfo.

[ximion]

I still would much rather want to add a way to do a trusted and verified download automatically, instead of linking the user to some random page on the internet where they would have to search for a way to download some random executable.
I'd consider adding such a URL an antifeature.
I would be open to adding a way to automatically fetch these things, e.g. by using the new release artifacts.

[ximion]

This is being discussed at #154 already as well.