Move PaletteModel to ui/, we need it on the client for the property editor.
Move property editor to ui/ for re-use in the client.
Make network/ part of a general purpose lib for common non-UI components.

There's more we have to share than just the pure networking code.
Move MethodArgument to common/ for reuse in NetworkObject.
Don't assert when receiving row removal notification for not loaded nodes.

Same fix is already in there for newly added rows.
Move plugin manager to common/, also needed in the client.
Move about plugins dialog to ui/, also used in the client.

Still needs some work though, it's internal models should reflect the
content of the server-side ToolModel instance, rather than the state of
local plugins.
Remove unused includes.