bump version to 3.9.0
gweather, invest: use GNetworkMonitor

 * Rather than talking to NetworkManager over D-Bus, use GNetworkMonitor
   to monitor the network state.

Signed-off-by: Sebastian Geiger <lanoxx@gmx.net>
Remove data parameters from callbacks
invest-applet: fix debug mode
invest-applet: import correct PanelApplet version
invest-applet: update preferences dialog