overviewControls: Remove slide transitions before setting value manually

Remove transitions of the slide-x property of the layout manager before we set the property to a fixed value, otherwise the transitions might still be running and change the value after we set it.

Before !708 (merged) this was meant to fix a bug, now it's just an improvement: The workspace slider remains hidden when opening the overview right after switching the workspace, it happens because "active-workspace-changed" starts an animation to update slide-x to 0 (_getSlide() returns 0 because the slider isn't visible), then after opening the overview slide-x is set to the correct value inside pageEmpty() while the animation is still ongoing, setting the value to 0 in the end.