minor refactor: Move Tab management into its own file

[zadjii-msft]

I think we can all agree that TerminalPage.cpp is an unruly beast of a
file. It's got everything. It does everything. It can sometimes be a bit
hard to work with, because of simply how big it is. This PR tries to
alleviate this by making TerminalPage.cpp just a little smaller. It
does so by moving pretty much everything related to tab management into
its own file, TabManagement.cpp. These methods that have moved are all
the same as they were before, and they're still members of
TerminalPage. But now they're all in one place.

I tried to move all the references to _tabs in TerminalPage.cpp, but
there's still a few that I left behind. Mostly because I felt that
moving those would be too gnarly a code change for an otherwise simple
cut&paste PR.

There are a few new methods I introduced:

• _TabDragStarted and _TabDragCompleted: These were lambdas before,
  promoted to full methods.
• _DismissTabContextMenus: Remove all the right-click context menus
  from the tabs
• _FocusCurrentTab: This one's a bit trickier, we were actually doing
  this in a few different places, so I tried consolidating.
• _HasMultipleTabs: This doesn't need explaining.
• _RemoveAllTabs: Really, just encapsulation for the sake of removing
  a _tabs from TerminalPage.cpp
• _ResizeTabContent: Really, just encapsulation for the sake of
  removing a _tabs from TerminalPage.cpp

In the future, some enterprising young soul could try promoting that
file to its own class, and hiding _tabs (and _mruTabs) inside it.
Probably would need to take a reference to TerminalPage's _tabView and
_newTabButton. I'm not doing that right now, because I already hate
the idea of the ...

920 additions and 847 deletions.

... I'm making you look at already.

Other thoughts

Some of the calls might be a little arbitrary - _OpenNewTab and
_CreateNewTabFromSettings probably should stay in TerminalPage? Or
at least elements of those might need to get split up better. Similarly
TerminalPage::_OpenSettingsUI stayed in TerminalPage.cpp, but it
does a lot of the same work as _CreateNewTabFromSettings. I'm not
saying this is the definitive places for these methods - it's code we're
working with, not stone ☺️

[carlos-zamora]

Yeeeeaaaaah. We should probably break up TerminalPage more haha.

Curious: does breaking it up help out with glomming & tab tear off at all? Maybe there's ways to shoe-horn some of that work in that effort?

If not, I say we file a CodeHealth task to figure out how to clean up TerminalPage a bit. Just to break up the responsibilities better.

[zadjii-msft]

Curious: does breaking it up help out with glomming & tab tear off at all? Maybe there's ways to shoe-horn some of that work in that effort?

maybe? We're way too far out to know for sure. Better encapsulation is always good though.

[DHowett]

clever

[DHowett]

Conflict resolution req'd.

[ghost]

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