Implement mimetype support reporting for MPRIS, start implementation of bulk of getters.

Implemented the following getters for MPRIS:

- CanQuit
- CanRaise
- HasTrackList
- Identity
- DesktopEntry
- SupportedUriSchemas
- SupportedMimeTypes
- Metadata
- CanPlay / CanPause / CanSeek
- CanControl
- PlaybackStatus

Implemented a koto_push_track_info_to_builder function that enables us to easily push KotoIndexedTrack as well as associated album and artist info to a GVariantBuilder for use in a GVariant for various getters.

Implemented a koto_update_mpris_info_for_track function that emits a signal for PropertiesChanged + "Metadata" when our track info changes.

src/playback/mimes.h

@@ -0,0 +1,28 @@
+/* mimes.h
+ *
+ * Copyright 2021 Joshua Strobl
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * 	http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+#include <glib-2.0/glib.h>
+#include <gstreamer-1.0/gst/gst.h>
+
+G_BEGIN_DECLS
+
+gboolean koto_bplayback_engine_gst_caps_iter(GstCapsFeatures *features, GstStructure *structure, gpointer user_data);
+void koto_playback_engine_gst_pad_iter(gpointer list_data, gpointer user_data);
+void koto_playback_engine_get_supported_mimetypes(GList *mimes);
+
+G_END_DECLS