preview-mode: Remove Preferences stubs

It has turned out that we don't need a Preferences page.
Therefore all stubs that were already in place
are removed again.
preview-mode: Include custom file first

When the optional custom file is included before the debug mode files
it is possible to use it for defining configuration variables.
Maybe one can do without inventing a tedious UI for defining preferences.
Merge pull request #208 from uliska/fixes

Fixes