Consolidate "standard" icons in one place (and make icon themes swappable)

[waddlesplash]

This follows up #10887.

There are a lot of relatively "standard" icons used throughout Haiku which would map cleanly to ones from Freedesktop's naming scheme (e.g. Up, Down, Go Back, Stop, etc.) in addition to the ones already used. If all these were consolidated and fetched from one place rather than being spread out across various resource files, it would be much easier to swap out icon schemes, if people wanted to do that. (As we support custom control looks and decorators, it seems to make sense to support this, too.)