New setting to force single column menus for sources & filters

The problem concerns potential multi-columns menus larger than the screen width that can be created by Qt - especially for the Alphabetical filters menu.

A new advanced option ForceSingleColumnMenus is added in the General Settings to force the sources/filters menus to be presented as a single scrollable column to ensure that all the filters are selectable on low resolution screens. Option is disabled by default.

Side effect: also fixes the ugly presentation of General Options.

For info, General Options used to look like this: