Further reduced duplication of packet sending code in FreeEmsComms

RequestClass is now much more explicit about packet type and properties,
this allows a sendPacket function that only takes a RequestClass, and
operates on it properly.
TableView3D: Disabled tracing checkbox until tracing is fixed properly
MainWindow: New plugin manager to add a UI for loading custom plugins

Plugins are now defined by a json file in the plugins folder, without this
json file, the plugin manager will not pick up the plugin.
Plugins: Addition of plugin definition file for FreeEMS