dshowvideosrc: add device-index, add GstDeviceMonitor, and several fixes

Submitted by Joshua M. Doe

Link to original bug (#797338)

Description

Created attachment 374042
0001 - dshowsrcwrapper: add debug category for general dshowsrcwrapper

It seems the DirectShow source dshowvideosrc hasn't seen much attention lately. The plugin winks provides ksvideosrc which supports most modern video sources, that is devices supporting the USB Video Class (UVC). However there are still several sources which are stuck with DirectShow, which I happen to be stuck using, so I worked on it a bit to make it usable for me.

They can be seen here:
https://github.com/joshdoe/gst-plugins-bad/tree/dshowsrc

There are 11 patches, should I upload them all here? I already did some squashing, but perhaps could go further to limit the number of patches.

Patch 374042, "0001 - dshowsrcwrapper: add debug category for general dshowsrcwrapper":
0001-dshowsrcwrapper-add-debug-category-for-general-dshow.patch