Windows thumbnail shell extension

[cow54787]

Description

Adds drawpile_shell_ext.dll on windows. This is a shell extension that provides thumbnails for .dpcs files.
The WIX installer has been updated to register the DLL as a shell extension.
The code is self contained, only relying on windows libraries (WRL for COM) and sqlite3. WIC (Windows Imaging Component) is used to parse the JPEG thumbnail inside the .dpcs file.

The PR also contains a simple test exe that loads the provider via COM and outputs a single thumbnail. I used it to test the shell extension and use a debugger, without having to rely on the file explorer.

The extension can be expanded in the future to support more file formats or other windows integration.

Formalities

• Did you write the code yourself or was any AI-assistance or similar used? If it's the latter, describe what.

I did use LLMs to understand various windows APIs.

[askmeaboutlo0m]

Looks good, only the ORDER BY snapshot_id DESC is functionally relevant, the rest is whatever.

SPDX-License-Identifier commands would also be nice to make it explicit. Just // SPDX-License-Identifier: GPL-3.0-or-later or // SPDX-License-Identifier: MIT or some other compatible thing at the top is fine. The Microsoft thumbnailer samples are MIT-licensed.