Turn tabHistory into a terminalDisplayHistory

Hitting Ctrl-Tab will now walk you through all opened terminals,
whether they are in split views or on different tabs.

Notes for Tomaz:

• if you don't plan to use it somewhere else, I guess we can remove

ViewManager::focusAnotherTerminal

• there might still be some work to do with the history when detach

is implemented.

• don't hesitate to comment if you like the feature as it is now,

we can discuss a better behavior if not.