Allow non-installable plugins in the plugin manager

[psifertex]

Feature request for the plugin manager:
Allow plugins that cannot be auto-installed. Needed for native plugins such as C/C++/Rust and would also help with plugins that aren't installed as stand-alone subfolders such as Lighthouse

[psifertex]

This was actually solved several releases ago, I believe in 4.2 @ElykDeer added support for view_only plugins.