Raise ShortcutActions with the sender (tab, control) context (#15773)

This PR's goal is to allow something like a `Tab` to raise a
ShortcutAction, by saying "this action should be performed on ME". We've
had a whole category of these issues in the past:

* #15734
* #15760 
* #13579
* #13942
* #13942
* Heck even dating back to #10832

So, this tries to remove a bit of that footgun. This probably isn't the
_final_ form of what this refactor might look like, but the code is
certainly better than before.

Basically, there's a few bits:

* `ShortcutActionDispatch.DoAction` now takes a `sender`, which can be
_anything_.
* Most actions that use a "Get the focused _thing_ then do something to
it" are changed to "If there was a sender, let's use that - otherwise,
we'll use the focused _thing_".
* TerminalTab was largely refactored to use this, instead of making
requests to the `TerminalPage` to just do a thing to it.

I've got a few TODO!s left, but wanted to get initial feedback. 
* [x] `TerminalPage::_HandleTogglePaneZoom`
* [x] `TerminalPage::_HandleFocusPane`
* [x] `TerminalPage::_MoveTab`


Closes #15734

src/cascadia/LocalTests_TerminalApp/TabTests.cpp

@@ -517,7 +517,7 @@ namespace TerminalAppLocalTests
 
         Log::Comment(NoThrowString().Format(L"Duplicate the first pane"));
         result = RunOnUIThread([&page]() {
-            page->_SplitPane(SplitDirection::Automatic, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
+            page->_SplitPane(nullptr, SplitDirection::Automatic, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
 
             VERIFY_ARE_EQUAL(1u, page->_tabs.Size());
             auto tab = page->_GetTerminalTabImpl(page->_tabs.GetAt(0));
@@ -535,7 +535,7 @@ namespace TerminalAppLocalTests
 
         Log::Comment(NoThrowString().Format(L"Duplicate the pane, and don't crash"));
         result = RunOnUIThread([&page]() {
-            page->_SplitPane(SplitDirection::Automatic, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
+            page->_SplitPane(nullptr, SplitDirection::Automatic, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
 
             VERIFY_ARE_EQUAL(1u, page->_tabs.Size());
             auto tab = page->_GetTerminalTabImpl(page->_tabs.GetAt(0));
@@ -857,7 +857,7 @@ namespace TerminalAppLocalTests
             // |   1    |   2    |
             // |        |        |
             // -------------------
-            page->_SplitPane(SplitDirection::Right, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
+            page->_SplitPane(nullptr, SplitDirection::Right, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
             secondId = tab->_activePane->Id().value();
         });
         Sleep(250);
@@ -875,7 +875,7 @@ namespace TerminalAppLocalTests
             // |   3    |        |
             // |        |        |
             // -------------------
-            page->_SplitPane(SplitDirection::Down, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
+            page->_SplitPane(nullptr, SplitDirection::Down, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
             auto tab = page->_GetTerminalTabImpl(page->_tabs.GetAt(0));
             // Split again to make the 3rd tab
             thirdId = tab->_activePane->Id().value();
@@ -895,7 +895,7 @@ namespace TerminalAppLocalTests
             // |   3    |   4    |
             // |        |        |
             // -------------------
-            page->_SplitPane(SplitDirection::Down, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
+            page->_SplitPane(nullptr, SplitDirection::Down, 0.5f, page->_MakePane(nullptr, page->_GetFocusedTab(), nullptr));
             auto tab = page->_GetTerminalTabImpl(page->_tabs.GetAt(0));
             fourthId = tab->_activePane->Id().value();
         });