add tab UI indicator of whether a private tab is running Tor

[diracdeltas]

If a user opens a bunch of private tabs and turns the tor switch off in some of them, it gets confusing to remember which tabs were Tor ones and which ones weren't. We should add some iconography to the tab to show that it's a Tor tab (similar to the icon for private tab and session tabs).

cc @bradleyrichter @rossmoody for an icon we can use

TEST PLAN:

1. open tor tab
2. you should see a 'Tor' icon in the tab
3. the 'Tor' icon should not appear on any non-Tor tabs

[alexwykoff]

We'll need a bit of a tighter icon for use in the tab control @rossmoody

[diracdeltas]

fixed by 6a5e8a9 for now

[srirambv]

Verified on Windows 10 x64 using

• 0.23.12 - 88f6f07
• Muon - 7.1.1
• libchromiumcontent - 67.0.3396.87

Verified on macOS 10.13.4 x64 using the following build:

• 0.23.12 88f6f07
• muon: 7.1.1
• libchromiumcontent: 67.0.3396.87

Verified on Ubuntu 17.10 x64

• 0.23.14 - f4da855
• Muon - 7.1.1
• libchromiumcontent - 67.0.3396.87