Battle backgrounds from GBA Sprites Pack

[danaYatsuta]

Incorporates these backgrounds into the game, as well as merges simplistic battle intro to display them correctly.

TODO:

• Fix text in bottom third of the screen appearing immediately upon start of encounter instead of when transition is complete
• Fix no option initially selected when opening options menu on 2nd screen
• Sort out fishing/surfing encounters bgs (currently all fishing encounters display lake bg, even when it doesn't make sense (fishing on sea routes, fishing in caves, probably some other instances); surf bg isn't implemented yet but will have the same issue. generally, surf and fishing should share bg)
• Same as above for beach/desert
• Blue window in indoors with Dewford gym palette