debug: fix the gui for smaller fonts

Using a font pointsize different from 11pt has a consequence that
fontWidth (in pixels) is not an integer anymore.
debug: fix a string overflow in comments
debug: dont update asm view in the start_hook()

The curpc can be in the middle of an instruction being decoded, so
it is not pertinent to recompute the disassembly starting from
this address.
debug: fix the avl trees logic

The ancestor heights of a node were not updated properly.
debug: better handling of duplicate memory access

Don't remove and reinsert nodes that correspond to a duplicate memory
access, because tree rebalancing kills performances. Update the node's
element content instead.
debug: fix wrong initial m_topleft.x value in memory view