Adding support for multiple desktop files for vscode

[tyvsmith]

Some applications, like VSCode & VSCode Insiders generate several .desktop files. For example, one to handle URL scheme.

This has been modified to copy all app*.desktop files into the build directory.

Here's a demonstration of this working. https://github.com/tyvsmith/VSCodeInsiders-AppImage

[tyvsmith]

Any interest in merging this, so the marketplace version supports this usecase? I'd rather not maintain a fork long term.