Bluetooth tethering

Introduce bluetooth network tethering support into the elementary UI.

Bluetooth tethering is a great on-the-go solution for mobile networking. Advantages of bluetooth tethering are:

- it uses significantly less battery than wi-fi tethering
- it is initiated from the device being tethered

The only disadvantage is the transfer speed, which is irrelevant on most mobile networks anyway.

The tethering UI should include:
- the possibility to control bluetooth network interface quickly (in switchboard-plug-network and wingpanel-plug-network)
- monitoring of the bluetooth networking (much like wifi, on wingpanel)
- auto-reconnect after suspend or bootup if the user so desires (to elliminate network setup hassle completely.)

Prior work: Bluetooth Manager (blueman) which uses bluez which we already have, which gives you standard networking interface:
```
bnep0: flags=4163 mtu 1500
inet 192.168.44.34 netmask 255.255.255.0 broadcast 192.168.44.255
inet6 fe80::5cfd:3cbd:9c1d:9a77 prefixlen 64 scopeid 0x20
ether 80:5e:4f:85:33:04 txqueuelen 1000 (Ethernet)
RX packets 5826 bytes 4322982 (4.3 MB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 6276 bytes 751559 (751.5 KB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
```

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