Support D-Bus action activation

[Antolius]

Problem

GAction registered on the application level can be remotely activated over D-Bus. This is currently supported by the Applications Menu, but not by Dock. The current Dock parses the .desktop files manually and extracts the exec line from them, which it then executes directly. This bypasses the mechanism for D-Bus action activation.

Proposal

Support activation of actions over D-Bus.

Prior Art (Optional)

Applications Menu supports this by virtue of using the DesktopAppInfo.launch_action method. See examples:

1. In the context menu
2. In search results

I made a little app that can be used to demo this:

example-of-action-activation-from-applications-menu.webm

[Antolius]

This feature request is motivated by the discussion on Document actions handling for launchers #180 issue over on developer docs repository.