cellayout: Investigate the possibility to cache the grid in AbstractGridLayout

[alexanderpann]

loadGrid is called for calculating the ascent, maximum/minimum inner size etc but I think the grid could be cached for ILayoutableContainer+size constraint key pairs. I didn't work for me for some reason but it would speed up the celllayout language a lot as I noticed with the profiler. One use is is the richtext language which is used by com.mbeddr.doc where the relayouting can be very slow when typing a lot of letters very fast.