sidebar: Fix unexpected switching to category page when clicking on separator
The main application item is supposed to be padded by the separator on the right. But in fact it has no clipping on its own, so if its subpage is sticking to the right -- you can still click it, and it will become focused and active as far as ColumnView is concerned. To fix this, prevent any interactions with the area covered by the separator by slapping a MouseArea on top of it.