Fixes for low FPS on slower systems during battles

[VannevarXbox]

Window_BattleStatus::Refresh was being called for every frame during a battle, triggering a full re-draw of the battle status window. This was causing very low framerate at some points on slower systems:

#3494
#3497

These changes improve battle framerate to a playable level on original Xbox. I've tested them with RM2k, custom test games for battle types A, B, and C (both large and small windows), the Maniac test game, and the game Ara Fell which was mentioned in the source.

It's my first pull request, so hopefully I didn't do anything too dumb! I'd be happy to answer any questions or make any needed changes.

• Refactored Window_BattleStatus::Refresh and Window::BattleStatus.RefreshGauge to update incrementally, or at least to skip redrawing content when not needed.
• The battle status window is no longer always cleared by a call to Refresh(), it only clears and redraws completely if the party actors change.
• In order to accomplish the incremental updates, Window_BattleStatus keeps a cache of the displayed values for each item, updating the graphics if those values change.
• The UIs for RM2k3 battle type B (alternative) and C (gauge) both have overlapping content, meaning multiple items need to be redrawn if one of the overlapping items is redrawn in the UI, so further optimization would be possible there.
• The function DrawGauge, which draws the gauges for battle types A (traditional) and B (alternative), has been moved from Window_Base into Window_BattleStatus, as that seems to be the only code that uses it.