Merge pull request #1 from jonnius/master

DE German Translation by Jonatan Zeidler
Fix: preference dialog: some combobox text remains untranslated.
Make preference mouse clicks combobox wider to better fit preference dialog.
Minor code clean-ups.
Adjust text to better look in different languages.
Preferences: addLabel allows for additional arguments to simplify its usage.
Add contributors to Welcome page of preference dialog.
Updated Russian translation.
Thumbnails dark focus/hover frame if thumbnail panel is light.
Version 8.
Default, min and max window preview reduced thumbnail panel opacity changed.
Preferences: allow binding notebook/page sensitivity to a key.
Ability to switch thumbnail panel off.
Ability to bind visibility instead of sensitivity.
Messages indicating why options are disabled when the option responsible for this is on another page.
Track all apps: not only on current workspace.
Remove autohideshow property of trackerapp: assume it is true.
Remove adding metaWindow in trackerApp _init: we do not pass it in anymore, anyway.
Move state from tracker.apps and tracker.windows to TrackerApp and TrackerWindows classes.
Show optionally icons from all workspaces.