New Preferences Dialog

[vkbo]

This PR completely rewrites the Preferences dialog.

Changes include:

• A new form layout that provides more helpful (I hope) text to the user. Design inspired by Gnome Tweak tool. One option per row in a two column layout.
• The checkbox has been replaced by an Android style switch.
• Connected the previous unreachable option to enable/disable full path in document header.
• Split all options over more tabs.

[johnblommers]

I like it! There is spit and polish in evidence. The dialog is friendly. It invites the user to explore.

[vkbo]

Thanks! I will redesign some of the other dialogs too.

I'm cleaning up bits and pieces before I start doing release candidates for 1.0. I will add more tweaks and improvements the coming weeks. Will probably do a 0.5 release first though due to the major rewrite of index and outline.