Meter rendering & correctness fixes.

Fixes:
	+ Did not respect the margin and spacing resources.
	+ There was a lot of (hacky) code to try to use all of the meter graphic area
		without exceeding it; not always successfully.
	+ bitmeter.cc did not handle narrow windows well.
	+ Inconsistency in how the fieldmeters*.cc handle total_ = 0.0
	+ fieldmeterdecay.cc didn't always draw updates to the decay graph.
	+ fieldmetergraph.cc didn't handle setting/changing the number of fields.

Fix & enhancement:
	+ fieldmetergraph.cc now automatically detects how much history to keep.