Allow using actions in the external preview

When using Vala, the code and preview runs in a separate process.

So far, we were unable to use actions there so we had to disable/downgrade demos for Vala. See

• https://github.com/sonnyp/Workbench/pull/101/commits/2df37308f602f93ae2d2a7a73da369922a45a362
• https://github.com/sonnyp/Workbench/pull/101/commits/145f37cd6e1277019692de0358886d5976259da4

This ticket is about enabling the usage of actions in library entries when using Vala.

See the previous description below and comments for an analysis of the issue.

Since then, the architecture changed a little bit and the external process isn't a GApplication anymore. Instead Workench and the external process communicate via a direct dbus connection instead of via the session bus.

See https://github.com/sonnyp/Workbench/pull/184/files#diff-24cdf4b8c7c2fbd1c6b43e8b7bcc877d14cae496dd01e82eb212b6d5b4c68d59

Please note that Flatpak apps cannot have multiple dbus targets. https://github.com/flatpak/flatpak/issues/1902

Todo

• Permit usage of GActions in Vala examples
• Upgrade Vala examples to use GActions like the js ones
  • Toasts
  • Notifications

Start with the external-process architecture described in the linked Workbench pull request #184 and compare the Vala examples with their JavaScript counterparts. Trace how the direct D-Bus connection handles actions; done means Vala library entries can use GActions and the Toasts and Notifications examples are upgraded.