Ported autotools to Meson. Closes #106.
Includes building gstreamer-sharp and gtk-sharp via Meson wrap files.
Also Updated build and runtime dependencies.

Convert README to MD

See merge request !8

Closes #176

The gstreamer backend now uses gstreamer-sharp instead of the custom C
bindings. This fixes #184 and #189.
Additionally, Gnome Subtitles now uses GLibSharp/GtkSharp instead of the
original gtk-sharp binding which haven't been updated in a while. The
new bindings will be bundled inside the Gnome Subtitles source tree. For
now, to build Gnome Subtitles, gstreamer-sharp (and its gtk-sharp
subproject) must be built and all dll files places in GS's build dir.

Fixes #185 Version 1.7 does not start on Fedora 35 beta
Fixes #188 Version 1.7.1 causes an exception in Ubuntu 18.04

Fixes #185 - Version 1.7 does not start on Fedora 35 beta