switch for bandwidth detection

[totaam]

See #999, #1855 and #1911.

We should make it easier to disable bandwidth detection, from the client.
Not sure if we should turn off detection if the server or client supplies a limit already.

[totaam]

Mostly done in r19878 + r19880, the server and client can set the detection on or off (still defaults to on):

xpra attach --mmap=no --bandwidth-detection=no

And the value can be seen with:

$ xpra info | grep bandwidth-limit.detection
client.bandwidth-limit.detection=False

[totaam]

r19905 adds a "Turn off" option to the congestion notification actions.

It would be nice to add a similar option to the client's system tray menu, but this would require:

• forwarding the flag from server to client via "settings-changed" mechanism
• syncing that with a new menu-entry
• augmenting the "bandwidth-limit" request packet with this new flag to allow the client to toggle at will

[totaam]

Too late for 2.4, moving the UI bits to #1972

@maxmylyn: FYI, this new switch makes it easier to rule out problems with the bandwidth detection code.