Copy lens plugin from HV 4.5.
Backport PluginClientWindow class from HV 4.5.

It is used by the lens and other plugins, and bring the CV plugin
API closer to HV 4.5.
Port lens plugin from HV 4.5 to CV plugin API.

Add automake based build rules and adjust code to use the
plugin API availaible in Cinelerra CV.  Add dummy
functions for defaults handling.