Move media plug-in support to application side.

The media server tried to use node monitoring to dynamically add and
remove plug-ins, but it isn't that useful:
* When a plug-in is added, applications would have to query the media
server to get an up to date list of available formats. For example
MediaConvert populates its format menus on startup.
* When removing a plugin, if an app already had it loaded, there is not
much that can be done to keep it working.
* The list of plugins was not sorted by directories (user vs system
add-ons), so the directories were re-scanned to make sure user add-ons
were returned first, rendering the node monitoring less interesting.

Now, the format handling is done by each application. The node
monitoring is removed, instead the apps will scan the plugin directory
when first using the media kit classes. Restarting the application is
needed to update the media formats list.
Urpdate AddOnManager and FormatManager for Media Kit

* These classes were moved from Media Server, making the use of ports
and messages to communicate with the app irrelevant.
* Split in a separate commit to help git keep track of moved files.

Fixes #4893.