build: switch active projects in Banshee.sln to default to Gst#

Following the change in the build to default to the GStreamerSharp [1]
backend, the solution should be changed as well to build GStreamerSharp
by default, and not build Banshee.GStreamer by default, so developers
using an IDE build the official dependencies without manual tweaks.

[1] https://git.gnome.org/browse/banshee/commit/?id=6bad3f59526a519a2243752466b4d9a2ba8abc44