Redesigned master selection dialog

The master select dialog now only shows detected systems and components
for reference, and does not allow them to be selected. Accordingly, the
MasterSelect class only returns an interface, not a 3-tuple.

The MasterSelect class now also requires a list of interfaces. The plan
is that it will handle auto-detection of systems and components itself.
Added code to listen to heartbeats
Fixed bug with updating GUI

New auto detected systems/components were not actually displayed in the
GUI. Now they are added.
Added code to mavproxy to use the master-select dialog
Fixed syntax
Serial interfaces are now added to GUI immediately
Fixed GUI bugs
Removed old, commented out code
Hopefully fixed sync issue with threads accessing GUI
Second attempt to hopefully fix sync issue w/ GUI