Add a "Close duplicates" button to the tabs panel

[monofunc]

Adds "Close duplicates" button under "Select".

• The button sits in the selection bar next to "Close all"
• Two tabs are duplicates when they share the same URL
• The most recently used tab of each URL is kept
• Tabs shown in a window, tabs without a URL and WebExtension sessions are never closed

Closes #2001

Screenshots:

[svillar]

First of all thanks for your contributions, really appreciated. Before reviewing the patch itself I wanted to ask about similar features in other browsers. I have the feeling that many options could be added there "close the last 10" "close the recent 5" etc... there are many possible combinations. I believe we should only add something like this if it's a common pattern in other browsers otherwise other users could ask for their preferred action in their workflows and we would end up with tons of buttons.

[monofunc]

Sure:

• Firefox: https://support.mozilla.org/en-US/kb/tab-overview-menu
  (close other tabs, to the right, to the left, close duplicates)
• Chrome: close other tabs, to the right
• Safari: close other tabs, to the right, to the left

I don't think we need right/left, it would be confusing. "Close other tabs" can be implemented (excluding active sessions).

[svillar]

• https://support.mozilla.org/en-US/kb/tab-overview-menu

Right, more than dups which is only implemented in Firefox, I think it makes more sense to implement "Close other"

[monofunc]

[monofunc]

(jk, fair point, let's wait until someone besides me actually needs it)