Windows: copy and paste images through the Windows clipboard; show a second window

[danjboyd]

Fixes #168 and #169: two gaps in the Windows workflow audit. Both are in released builds.

#168: images don't cross the Windows clipboard.
GNUstep's gpbs bridges only text (gnustep/libs-back#246 would fix it, but the MSI uses MSYS2's stock gnustep-back). On Windows the app now uses the clipboard itself, through the new STWindowsIntegration.

• Copy writes the registered PNG format (Office, browsers and Teams keep transparency) and a 32-bit CF_DIB for Paint and the rest. It doesn't write the pasteboard as well: that would make gpbs empty the clipboard and offer it as text.
• Paste, Paste as New Image and Take Screenshot:
  • read PNG if it's there;
  • otherwise read the bitmap, drawn by GDI so any depth, palette or bitfields DIB works;
  • also accept image files copied in Explorer (CF_HDROP).
• Elsewhere: off Windows nothing changes. The functions return NO or nil there, and tests that substitute their own pasteboard keep it.

#169: a second window never showed.
NSTask starts processes with SW_HIDE on Windows. So the instance started by Paste as New Image, or by Take Screenshot over an open image, loaded its image and stayed invisible.

• On Windows it's now started with CreateProcessW instead.
• On all GNUstep platforms, an empty window takes a pasted image itself, as Take Screenshot already did, rather than being left beside a new instance.

Verified on Windows 11 (WinUITheme, dev build)

• Windows → app: a bitmap copied by another process enables the empty state's Paste, and pastes in place with the right colors.
• App → Windows: after Copy, the clipboard holds PNG and DeviceIndependentBitmap, and Windows adds Bitmap and DIBV5 itself. Both decode to the image; the bitmap was all black until the conversion stopped going through drawing.
• Paste as New Image over an open image: the new instance's window is visible.
• Tests:
  • New WindowsIntegrationProbeTests (9 tests, Windows only): DIB out (alpha, RGB), a round trip, 24-bit, top-down bitfields and 8-bit palette DIBs in, a truncated DIB refused, and command-line quoting checked against CommandLineToArgvW. They don't touch the real clipboard.
  • New testAnEmptyWindowTakesThePastedImageItself.
  • Full suite on Windows: all classes pass, with the same 4 skips as before.
• Not done here: the Snipping Tool overlay itself (it needs a person to drag), and the Linux and macOS builds. The non-Windows side is a stub plus the empty-window change.

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