(MemoryMonitorPanel) Compute colors once
(MemoryMonitorPanel) Cache the background, redraw on resize
(MemoryMonitorPanel) Cache the font
(MemoryMonitorPanel) Cache the strokes
(MemoryMonitorPanel) Cache more colors
(MemoryMonitorPanel) Cache the no-network image
(MemoryMonitorPanel) Repaint the background from paintComponent instead of the resize event

The resize event happens too late and not often enough, so that the
background was not redrawn often enough. With a simple size check in
paintComponent it is ok.
Merge pull request #532 from tlecomte/memorymonitorpanel

Memorymonitorpanel: better memory usage.