Auto-resize Settings to fit wide widgets

Add check for settingsTree width.  If it's not at the maximum width,
resize the Settings dialog to fit it.  Qt should keep the dialog
within bounds of the screen, and the user can always resize the
dialog if needed.

This fixes the Settings dialog squashing the settings tree with the
Sonnet spell-checking widget, Ubuntu 16.04 on en_US, and a 1920×1080
screen.