Detect video types more reliably

Under macOS, GLib.ContentType.guess() may actually return something like
public.avi (see https://en.wikipedia.org/wiki/Uniform_Type_Identifier#Apple_public_UTIs). get_mime_type() increases the chances of a proper identification.

Author: fnevgeny

src/classes/action/movie.vala

@@ -264,7 +264,8 @@ namespace pdfpc {
             }
             bool uncertain;
             string ctype = GLib.ContentType.guess(uri, null, out uncertain);
-            if (!("video" in ctype)) {
+            if (!("video" in ctype) ||
+                !("video" in GLib.ContentType.get_mime_type(ctype))) {
                 return null;
             }