Add block icon cache to pqMultiBlockInspectorPanel

This adds a cache for block icons to the multi-block inspector
panel. Previously, much time (and memory) was being spent on
creating icons for each block. Now, blocks that share the same
attributes (color and opacity) will use the same icon.

Change-Id: Ib81ac6880781cdd9777e6421ba5a0a00da09baed
Merge topic 'block-icon-cache' into next

61e116f Add block icon cache to pqMultiBlockInspectorPanel