Fix reading window title from _NET_WM_NAME.

XGetWindowProperty() returns 0 on success.

Closes #236
Add an option to display a tray icon.

Also implement "Minimize to tray" functionality.
Use specific monospace fonts as fallback on Mac OS X.

Qt (4.8.6) doesn't seem to be able to resolve the generic monospace
font family.

Closes #214
Coding style fixes.
Add Travis CI config.
Update translations.
Define QT_NO_DEBUG for build type None.

Debian sets the the build type to None for package builds.
Make sure we don't enable asserts there.

Closes #237
Enable debug mode for Travis CI builds.
Merge remote-tracking branch: upstream/master