Identify "plugin" applications by checking for custom .desktop property

This is an evolutive for the plugin system.

The idea is to remove plugin installation/updating from envision and instead relying on the user's preferred distribution system for installing them.

The proposed way to accomplish this is to parse installed .desktop files and look for a new custom property X-XR-Plugin=true to be set.

(note that the property name is still not final as of writing this, so feel free to leave a comment if you have a better suggestion)

Applications detected this way will be shown in the plugins list, with their appropriate Name, Comment and Icon. They will be disabled by default, and can be enabled with a switch.

Additionally, an X-XR-Plugin-Exec property can be added to instruct Envision (or any other future software that wants to use this) on a specific way it needs to be ran.

Installation/uninstallation will not be handled by envision anymore.

To run the enabled plugins under the new system, the existing mechanism can likely be adapted without much effort.