Speed up message cell text size evaluation and display

[fpillet]

Huge logs come with a price tag: performance degrades when loading large log files or resizing the window, as the software has to recompute the best size for message cells. Layout is currently slow because using NSString's boundingRectWithSize: which has to recreate a layout manager everytime we have to compute a size.

Solutions are:

• During cell size calculation, provide a layout manager where we just update the text, quickly compute its height and reuse the manager as much as possible
• Switch to CoreText for calculation and display.