Add tabnextmru bound to ctrl+7

[bovine3dom]

Related: #4580

press ctrl+7 repeatedly to navigate through previously used tabs. Any tab used within the last 10 seconds is ignored.

it works pretty well, I think. But it's a bit confusing that it messes up the order of the tabs in :tab with :set tabsort mru once you've finished.

Essentially what I want to do is:

1. flick through the 2-4 most recently used tabs to find the one I want
2. have ctrl+6 get me back to the tab I was first on, then use ctrl+6 repeatedly to switch between those two tabs.

I can do 1) but not 2) because it always becomes the last-but-one tab that I looked at when trying to find the tab I wanted.

I don't know why b[tpress <Tab> repeatedly then <CR>] is unsatisfying to me because in theory it should be a good enough solution.

Maybe when we have keydown/keyup binds I could have it so that:

1. hold down control
2. tap 7, :tab opens with first completion selected
3. tap 7 repeatedly to cycle selection
4. release control to accept selection and go to tab

But I'm not sure that is the real problem. Surely it can't just be the keybinds that are wrong...

[bovine3dom]

I think this won't work for people who already have browser binds