gdb/python: introduce gdb.TargetConnection object type

This commit adds a new object type gdb.TargetConnection.  This new
type represents a connection within GDB (a connection as displayed by
'info connections').

There's three ways to find a gdb.TargetConnection, there's a new
'gdb.connections()' function, which returns a list of all currently
active connections.

Or you can read the new 'connection' property on the gdb.Inferior
object type, this contains the connection for that inferior (or None
if the inferior has no connection, for example, it is exited).

Finally, there's a new gdb.events.connection_removed event registry,
this emits a new gdb.ConnectionEvent whenever a connection is removed
from GDB (this can happen when all inferiors using a connection exit,
though this is not always the case, depending on the connection type).
The gdb.ConnectionEvent has a 'connection' property, which is the
gdb.TargetConnection being removed from GDB.

The gdb.TargetConnection has an 'is_valid()' method.  A connection
object becomes invalid when the underlying connection is removed from
GDB (as discussed above, this might be when all inferiors using a
connection exit, or it might be when the user explicitly replaces a
connection in GDB by issuing another 'target' command).

The gdb.TargetConnection has the following read-only properties:

  'num': The number for this connection,

  'type': e.g. 'native', 'remote', 'sim', etc

  'description': The longer description as seen in the 'info
                 connections' command output.

  'details': A string or None.  Extra details for the connection, for
             example, a remote connection's details might be
             'hostname:port'.

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