Support XDG Actions

[rawkode]

Is your feature request related to a problem? Please describe.
I have my browsers configured with certain actions, as below. It would be nice if browsers could detect them and offer them as launchers.

[Desktop Entry]
Actions=link-handler
Exec=/nix/store/cfdzahy3vp1s3rqipdb57dbpijxpxqcb-vivaldi-6.8.3381.46/bin/vivaldi --app="https://github.com"
Icon=/nix/store/lvspxy14mz1h8mvqnlvs2nm7drjs0qf7-icon.svg
Keywords=gmail
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;text/mml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall
Name=GitHub
NoDisplay=false
StartupNotify=true
StartupWMClass=vivaldi-github.com__-Default
Terminal=false
Type=Application
Version=1.4

[Desktop Action link-handler]
Exec=/nix/store/cfdzahy3vp1s3rqipdb57dbpijxpxqcb-vivaldi-6.8.3381.46/bin/vivaldi --app=%U
Icon=/nix/store/lvspxy14mz1h8mvqnlvs2nm7drjs0qf7-icon.svg
Name=Link Handler
[Desktop Entry]
Actions=link-handler
Exec=/nix/store/cfdzahy3vp1s3rqipdb57dbpijxpxqcb-vivaldi-6.8.3381.46/bin/vivaldi --app="https://github.com" %U
Icon=/nix/store/lvspxy14mz1h8mvqnlvs2nm7drjs0qf7-icon.svg
Keywords=gmail
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;text/mml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall
Name=GitHub
NoDisplay=false
StartupNotify=true
StartupWMClass=vivaldi-github.com__-Default
Terminal=false
Type=Application
Version=1.4