Reimplement filtering in connection list. The filtering now works like a
matrix: you choose one of the connections types (Normal, VPN, Shared) and
optionally an interface / device on the left widget. If no interface is
selected then the connection list will connections for all interfaces.
If one interface is selected it will show only connections that can be
activated using that interface. This is how branch nm09 works except of
the connection types.
I have reimplemented the filtering to also remove some code that are not
needed or that can cause performance degradation.