This commit changes how we handle focus and tabstop-ness in TermControl
and its descendent controls.

The root of the issue that #4031 set out to investigate is that
TermControl hosts a bunch of other controls, but it's using event
tunneling to get first dibs on every bit of keyboard input sent its way
-- even for things that live inside it. This required us to hack around
and detemrine whether the input was intended for someone else before
eventually ignoring it or using it.

I believe the _correct_ thing for us to do is to direct user keyboard
input to a control that lives logically "inside" the terminal surface
and have that control hold focus and process keyboard events.

Herein, we introduce an empty and invisible UserControl to hold focus
inside the terminal surface. Everything I've tested works great here.

It's important to note that the UserControl is invisible. Unfortunately,
this means that it can't receive **mouse** input. Its parent, the
SwapChainPanel acting as the terminal surface, can.

I've gone through and unmarked things that should not show up in the
tabstop list and changed the focus parameters for things that should.

Doing this lets us kill TermControl's dependency on all private details
in SearchBoxControl, including the bit where we could ask "do you think
you should be focused?", by letting Xaml handle focus. We now listen to
more focus events and funnel focus where we think it belongs. For
SearchBoxControl, that means we focus the text field and continue to
select all the text in it when the SearchBoxControl gets focus.

Fixes #4031.
This is an alternative solution to #4723, so I reverted that fix.

This commit changes how we handle focus and tabstop-ness in TermControl
and its descendent controls.

The root of the issue that #4031 set out to investigate is that
TermControl hosts a bunch of other controls, but it's using event
tunneling to get first dibs on every bit of keyboard input sent its way
-- even for things that live inside it. This required us to hack around
and detemrine whether the input was intended for someone else before
eventually ignoring it or using it.

I believe the _correct_ thing for us to do is to direct user keyboard
input to a control that lives logically "inside" the terminal surface
and have that control hold focus and process keyboard events.

Herein, we introduce an empty and invisible UserControl to hold focus
inside the terminal surface. Everything I've tested works great here.

It's important to note that the UserControl is invisible. Unfortunately,
this means that it can't receive **mouse** input. Its parent, the
SwapChainPanel acting as the terminal surface, can.

I've gone through and unmarked things that should not show up in the
tabstop list and changed the focus parameters for things that should.

Doing this lets us kill TermControl's dependency on all private details
in SearchBoxControl, including the bit where we could ask "do you think
you should be focused?", by letting Xaml handle focus. We now listen to
more focus events and funnel focus where we think it belongs. For
SearchBoxControl, that means we focus the text field and continue to
select all the text in it when the SearchBoxControl gets focus.

Fixes #4031.
This is an alternative solution to #4723, so I reverted that fix.