Feature: Open a new tab by middle clicking the empty space in the tab strip

[TRUINGLol]

Resolved / Related Issues

• Closes Feature: Open a new tab by middle clicking the empty space in the horizontal tab control #3808

Steps used to test these changes

1. Opened Files, middle-clicked the empty strip between the "+" button and the caption buttons — a new tab opens and is selected. Repeated with several tabs open and with the window maximized.
2. Dragged the window by that same empty area, including snapping to the top and side edges.
3. Double-clicked it (maximize/restore), right-clicked it (system menu), used the minimize/maximize/close buttons.
4. Middle-clicked a tab (still closes it), the "+" button, the toolbar, the Omnibar and the file list — no stray tabs.
5. Repeated steps 1–2 with the app language set to an RTL language.

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The empty tab strip area is registered as a Caption region, so middle clicks there never reach the XAML tree. The non-client messages don't reach the main window either: WinUI routes them to a dedicated InputNonClientPointerSource child window and forwards only WM_NCLBUTTONDOWN and WM_NCRBUTTONUP up to the top level.

So this PR subclasses that child window via WinUIEx.WindowMessageMonitor and handles WM_NCMBUTTONDOWN / WM_NCMBUTTONDBLCLK with wParam == HTCAPTION, invoking the same Commands.NewTab the "+" button uses.

[CLAassistant]

All committers have signed the CLA.

[TRUINGLol]

If anyone is interested here is a summary of the resources that helped me understand how to solve this correctly (also alot chatgpt chats xD)

non client area
mouse
window n procedures
subclassing win
title bar winui

[yair100]

I can confirm this works as expected. That being said, I won't be able to review the code changes until next week.