correct QSettings handling

always use the three-argument form: s.value(key, default, type)
use type("") as the type for strings, this works in py2 and py3
default to False to not surprise the user
Update German translation

I mainly went through all 'fuzzy' entries and looked for emty msgid entries.

I did _not_ go through all untranslated documentation. This is a project of its own, I'm afraid.
preview-mode: Uncheck Checkboxes in initial state
preview-mode: Add annotate-spacing

This simply activates the built-in mode.
But it is a nice complement to the debug-notecolumns mode
Merge pull request #205 from uliska/translate-de

Update German translation
preview-mode: Add documentation

I have only added English documentation to the main file and didn't know
how to update the language settings files.
manually merge uliska-preview-mode
simpler help text generation
avoid structural markup in new translatable strings
improve formatting of help
pot update
parts of nl.po
typos
upd nl
cred, chlog, nl.po
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