Fix scrollbars on the directory tree and page tree elements

[enlight]

Overlay scrollbars look a lot nicer than regular scrollbars since they don't force elements within the scroll region to shrink to make room for the scrollbars. There is a Chrome setting to enable this style of scrollbar, though my previous attempt to do so was unsuccessful, try again after upgrading to Electron v0.36 (the relevant setting is probably set via the Blink flags/features when creating a BrowserWindow).