Update and clarify experimental/vte

[felipec]

I'm trying to figure out if any of the patches in experimental/vte are needed. It seems disable-bg-draw.patch is not needed anymore since upstream has it now, except with the name vte_terminal_set_clear_background. The code should probably use that instead.

I tried to update the patch alternate-screen.patch for the latest version of vte, and I managed to do it here, but I've no idea how to test it. Is it really needed?

I'm trying to gather all the important patches of vte in my repository: felipec/vte. Yours are in the tilix-patches branch.