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

Merged Joshua M. Doe requested to merge joshuadoe/gst-plugins-bad:dshowsrc_work into master

This merge request is for the remaining patches on #807 (closed). I've resolved most comments, except for one from @nirbheek about where to open the device. Not sure how GitLab workflow is supposed to work, should that discussion be had here or on the issue?

Edited by Nirbheek Chauhan

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