fix one scrolling issue (again)

[guiv42]

see:

• original issue: https://github.com/helge17/tuxguitar/issues/534
• db7b368f5577118e35007045946d6a5bccffbd26 first attempt to fix (KO)
• unexpected side effect: https://github.com/helge17/tuxguitar/issues/536

Detailed description of sequence leading to bug is available in commit db7b368f5577118e35007045946d6a5bccffbd26

Another attempt to fix, limiting the risk of side effect: Behavior of Tablature.updateMeasures is modified only after processing TGMouseMoveAction or TGMouseExitAction