Rename setAcceptsHoverEvents

setAcceptsHoverEvents was obsoleted in place of setAcceptHoverEvent in
Qt 4.4. They behave the same.
Merge pull request #1283 from simonsonc/rename-accept-hover-events

Rename setAcceptsHoverEvents
Do not render grid annotation if the lines/markers would be extremely dense

Otherwise drawing of huge amount of lines/markers can lock GUI
Decoration grid: fix symbol sizes + default cross size