grabbing has problem
can grab mouse and keyboard now, but can't receive event
can catch motion event now, but xgraber blocks main thread heavily
ensure grabpointer success; yield cpu when no X events
grab mouse and keyboard done
grab mouse and keyboard on showing
bugfix: XGraber's flag variables didn't get reset after menu dispeared
remove destroyForward and destroyBackward, using destroyWholeMenu destroySubs instead
bugfix: checked not memorized
bugfix: checkMarg flag not work