qtui: Properly fix the SettingsDlg layout

We've suffered from the page tree in the settings dialog being
squashed horizontally for a long time. A previous commit (65c463f0)
tried to fix this by calculating a minimum size and explicitly
resizing the dialog, but this didn't properly work at least on HIDPI
displays.

Instead, fix layouting properly by setting the correct size policy
for the tree widget, and removing both the afforementioned workaround
and a previous hack that tried to set the minimum width manually
(and failed due to a still-hidden widget reporting wrong size hints).