[Resolved] Crash on opening a video for second time. Plugins are released and
then initialised again before opening a new video stream.

[Resolved] "Highgui:: parameter not set" issue when opening the web cam video
feed is resolved. Now on web cam mode frameId is always -1.
Previous Button is disabled on web cam mode. Since this is irrelavant in this
mode.
PluginPassData reference is passed (local variable) as const PluginPassData&
Since the plugins are connected in a single thread this won't be an issue.
(emiting signals are synchronous in single threaded applications. Signal
won't return until all the slots connected are returned.)

And there is no need for **memory management** for the plugin developer.