Add AppVeyor Qt build for v5 shell

[DavidAGInnovation]

Summary

Add a v5-compatible AppVeyor configuration for the legacy Qt shell branch.

Problem

The v5 branch did not contain an AppVeyor configuration, so AppVeyor fell back to its default MSBuild mode and stopped before compiling:

No Visual Studio project or solution files were found in the root directory.

Changes

• Initialize the pinned libmpv and SingleApplication submodules.
• Install and verify the x86 OpenSSL 1.1.1 dependency expected by stremio.pro.
• Use the Visual Studio 2019 x86 toolchain.
• Invoke the existing Qt 5.15.2 qmake project and build it with nmake.
• Verify that the resulting stremio.exe exists.

Validation

• YAML parsed successfully.
• git diff --check passed.
• The Windows Qt build must run in AppVeyor.