[WizzardSK]

Small fixes for several open issues:

• Fixes https://docs.libretro.com/guides/download-cores/: Add a section how to manually download cores #912: Installing Cores gets a section on installing a core by hand from the buildbot, including the core info file.
• Fixes add "dynamic wallpaper" guide #300: the XMB guide explains dynamic backgrounds (setting, directory, file naming), instead of linking to the old Lakka wiki page.
• Fixes Gameboy Converter tool #954: the VBA Next and VBA-M pages point to the GBA 4gz/4gs/sav converter for their libretro VBA format .srm saves.
• Fixes add DAT links for the various MAME cores #394: MAME 2015 now links its XML DAT like the other MAME pages already do (MAME 2016 has no page yet).
• Update bounties page with new preferred bounty payment service #960: removes the remaining BountySource link from Quick Informations; the bounties page already says the service no longer works.

Menu labels are taken from current RetroArch master.