External UI is unusable

[riban-bw]

Some LV2 plugins provide their own GUI that they can host themselves, not dependant on the LV2 host. This is launched with the show_external_ui call. An example LV2 with this feature is synthv1. The LV2 plugin needs to have its idle function called peridically to allow it to perform functions such as update its GUI. This only happens in mod-host when there is a socket connection and a command is processed, i.e. the LV2 plugin's GUI is only updated after a mod-host command over socket. It won't work at all with the CLI.

The GUI does function, i.e. changes made on the GUI seem to take affect but the GUI is not updated with those changes until mod-host receives a command.

The call to effects_idle_external_uis is within the socket recv handler. For this to work we probably need effects_idle_external_uis to be called more regularly in its own (or another appropriate) thread. We could use poll instead of socket.recv to allow timeout and subseqent effects_idle_external_uis call but this does not fix it not working in the CLI (interactive) mode.