Implement a rudimentary core/dll download form

[Morilli]

• (resolves) #3505

This implements a very simple download form that can be used to download additional files into the dll directory, mainly intended to be used for large core downloads, such as the mame or encore cores. This, in addition to excluding those files from the release zips can help bring the initial download size of bizhawk down.

How it currently looks like (accessable from Tools->Download cores):

PRing for visibility and feedback. I think this could work as is, but probably needs additional support from the excluded cores to not feel bad when a user tries to load a ROM for a core that isn't downloaded and is then presented with an undecipherable error message.

Disclaimer: I do not take responsibility for the diff in Mainform.Designer.cs.

Check if completed:

• [x] I have run any relevant test suites
• [x] I, the committer, have read the licensing terms for contributors (last updated 2024-03-20) and am compliant