dont include the debug-layout-options.ily when no single debug option is used
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.
how the preview mode tool widget works a bit more clear

make checkOption just a inner function, it need not be a public method
Merge pull request #208 from uliska/fixes

Fixes
Update fr.po
added MainWindow.aboutToClose and aboutToCloseLast signals
Merge pull request #209 from philmassart/patch-3

Update fr.po