Fixes 658: Error during startup for windows missing plugin

This is my proposed fix for the issue. 

I have moved the calls to get_default_device() methods into the affected plugins' get_videoinput_bin() (get_audioinput_bin() for Pulsesrc) methods. This ensures that get_default_device() is only called when the plugin actually gets used, instead of during freeseer initialization.

Previously, the Yapsy PluginManager was loading the plugin module during initialization, which was causing get_default_device() to run, which in turn calls a get_devices() method. In the case of Pulsesrc audio, get_devices() attempts to probe the plugin which doesn't exist in windows.