Add GsPluginJobUrlToApp and use it to replace gs_plugin_url_to_app()
These commits are from @mcrha, split out from !1875 (closed), rebased, reviewed and slightly tweaked by me to fix review issues. I’ve made one significant change, pushed as an additional commit. It will need review:
- Rewrote url-to-app implementation in
snap
plugin to be fully async
Helps: #1472