Make all `Form`s inherit `FormBase` instead

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This was always intended to become a universal base class. The goal was to have a way to inject anything into every form, if that ever became necessary—but it's since become clear that proper input handling involves passing the focused window's inputs (or a subset thereof) to the root, so this would have multiple benefits.

Start by reviewing the listed forms under src/BizHawk.Client.EmuHawk, including ArchiveChooser.cs, the config forms, movie forms, RetroAchievements forms, and tools forms, then locate FormBase and inventory remaining Form subclasses. Done means all Forms inherit FormBase and the focused window's inputs can be passed to the root as described.

Written by the indexing model from the issue text.

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