Fix two-pixel gap at the bottom of a maximized Terminal window

[reflectronic]

When a window's client area fully covers a monitor, Windows considers it a "fullscreen" window and drops the taskbar's always-on-top status. If Terminal is configured to hide the title bar, maximizing the window causes its client area to fully cover the monitor, triggering this behavior. This prevents the auto-hide taskbar from appearing when the user hovers over the edge of the screen (#1438).

To work around this, Terminal previously shrank the client area by 2 pixels if an auto-hide taskbar was detected. However, this creates a visible gap where the desktop or other applications show through the maximized window.

To fix this correctly, we set the "NonRudeHWND" property on the window, which tells Windows to never treat Terminal as a "fullscreen" window. Of course, when the user enters actual fullscreen mode, we remove this property. This lets us remove the workaround and let Terminal's client area fully cover the monitor when maximized.

Fixes #12959

[DHowett]

this is much smarter. It looks like IslandWindow has support for fullscreen mode as well; should we propagate this change there, or is it not subject to the same issue?

(fwiw: that is the one we use when "show tabs in the titlebar" is turned off)

[DHowett]

/azp run

[azure-pipelines]

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