This pull request introduces Microsoft.Terminal.Core.Color as an
alternative to both Windows.UI.Color and uint32_t/COLORREF in the
TerminalCore, ...Control, ...SettingsModel and ...SettingsEditor layers.
M.T.C.Color is trivially convertible to/from til::color and therefore
to/from COLORREF, W.U.Color, and any other color representation we might
need².
I've replaced almost every use of W.U.Color and uint32_t-as-color in the
above layers, with minor exception¹.
The need for this work is twofold.
First: We cannot bear a dependency from TerminalCore (which should,
on paper, be Windows 7 compatible) on Windows.UI or any other WinRT
namespace.
This work removes one big dependency on Windows.UI, but it does not go
all the way.
Second: TerminalCore chose to communicate mostly in packed uint32s
(COLORREF), which was inherently lossy and dangerous.
¹ The UI layers (TerminalControl, TerminalApp) still use
Windows.UI.Color as they are intimately connected to the UWP XAML UI.
² In the future, we might even be able to *use* the alpha channel...
## PR Checklist
* [x] I ran into the need for this when I introduced cursor inversion
* [X] Fixes a longstanding itch
## Validation Steps Performed
Built and ran all tests for the impacted layers, even the local ones!
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