TreeBrowser: Port spawning external commands to the spawn API
Port "Open Externally" and "Open in terminal" actions to the spawn API for better support on Windows. Also remove unnecessary hard-coded "cmd" command on Windows and use the configured command instead.
Closes #556.
@medovina Did you had a chance to cross-read that PR?
Since this has been confirmed working in https://github.com/geany/geany-plugins/issues/1194#issuecomment-1279998807, I would like to merge this in a few days.
I apologize for the long delay in replying. I was the maintainer for the treebrowser plugin a few years ago, but I no longer use Geany and can no longer review treebrowser patches. I just filed a pull request to remove myself as maintainer. Best of luck with this patch and further work on the plugin.