shellDBus: Change AcceleratorActivated signature

Adding new parameters to the signal currently will break keybindings
until gnome-settings-daemon is updated to the new API as well.
Put additional parameters into a dictionary instead to make future
extensions easier.

https://bugzilla.gnome.org/show_bug.cgi?id=711682
shellDBus: Add mode parameter to AcceleratorActivated signal

This will allow g-s-d to handle actions differently based on the
current mode - namely, allow the power button when locked, but
make sure to never show any dialogs in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=711682