Hotfix/slider block click on transition

This fixes an unwanted behaviour that happens when the user clicks on menu items when the menu is still in a transition. The OverlayPaneSlider has a flag that its parent read to know if they let the user interact with the menu or not. Simply put, it forbids any kind of operation from the MenuBarContainer onto the OverlayPaneSlider if the slider is in a transition. I also cut down the transition duration when scrolling up to clear the flag as soon as possible.

Also I cut down duplicate functions to factorize a bit of code in the OverlayPaneSlider.

EDIT: there are other horizontal scrolling problems with the OverlayPaneSlider (#23 (closed) ) so i'll try to fix this one before merging.

EDIT2 : fixed