Drop in 'Web Look And Feel'

It currently does not look good

closes #95
Improve looks
Redesign drastically
Move tab icons in separate folder
Improve menu button look
Improve menu
Make the context editor look really sexy
Remove selection if clicking outside context editor
Remove status bar at least visually

Maybe replace the progress indication with a spinning wheel next to the corresponding view's title?

See also #96
Replace conexp icons with hand-made for context editor
Remove unnecessary batik dependencies from pom.xml

closes #29
Give the main menu some functionality