GUI: propagate signals from nested envelopes to the topmost MessageView
GUI: refactor address rendering into a special function
abook: implement a method for querying the list of known contacts based on the mail address
GUI: Decorate contacts based on whether they's in the address book
Add missing copyright details
Add an accessor for retrieval of e-mail URLs from MailAddress
Extract QUrl -> MailAddress parsing into an extra function
GUI: utility function for converting SVG images to a downscaled image usable within QLabel
GUI: adding contact icons

- contact-unknown is Oxygen's actions/im-user.svgz
- contact-known is Oxygen's actions/favorite.svg
Add icons for management of addressbook entries from the MessageView

refs #270
Fix typo in comment
GUI: tell the user that "close" actually means "minimize to systray"

Thanks to Franz Fellner <alpine.art.de@gmail.com> for his bugreport and to
http://wayback.archive.org/web/20090527102002/http://blog.beef.de/2008/01/12/qmessagebox-widgets/
for a nice idea on how to hack QMessageBox with this feature.

refs #67
sources for the abook GUI
qmake: remove stuff which isn't available anyway
UI: split the UI definitions into two files
Split be.contacts into an application and the GUI class
qt5: unbreak the build of be.contacts
wip: porting the AbookAddressbook to use be.contacts
Preparing for standalone libraries
cleanup