Use HeaderBar
Replace classical menu bar with application and gear menus

Also add new icon and update setup script
Remove unused application preferences dialog
Remove specialized menu items for creating new files of different types
Draw icons based on mime-types, not internal Oxalis types
Remove Oxalis file-type enum, move TreeModel columns enum to SiteStore class
Detect if application was installed and set path to resources accordingly
Update site settings dialog
Update .gitignore
Add UI definition file to the setup script
Rework configuration system
Use fallback values for options in settings dialog
Update README
Reorganize meta-information in __init__.py and move resource paths to resource.py
Remove SmartyPants from the source tree