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.c

@@ -0,0 +1,66 @@
+/* mimes.c
+ *
+ * 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.
+ */
+
+
+#include <glib-2.0/glib.h>
+#include <gstreamer-1.0/gst/gst.h>
+#include "../koto-utils.h"
+
+GHashTable *supported_mimes_hash = NULL;
+GList *supported_mimes = NULL;
+
+gboolean koto_playback_engine_gst_caps_iter(GstCapsFeatures *features, GstStructure *structure, gpointer user_data) {
+	(void) features; (void) user_data;
+	gchar *caps_name = (gchar*) gst_structure_get_name(structure); // Get the name, typically a mimetype
+
+	if (g_str_has_prefix(caps_name, "unknown")) { // Is unknown
+		return TRUE;
+	}
+
+	if (g_hash_table_contains(supported_mimes_hash, caps_name)) { // Found in list already
+		return TRUE;
+	}
+
+	g_hash_table_insert(supported_mimes_hash, g_strdup(caps_name), TRUE);
+	supported_mimes = g_list_prepend(supported_mimes, g_strdup(caps_name));
+	supported_mimes = g_list_prepend(supported_mimes, g_strdup(koto_utils_replace_string_all(caps_name, "x-", "")));
+
+	return TRUE;
+}
+
+void koto_playback_engine_gst_pad_iter(gpointer list_data, gpointer user_data) {
+	(void) user_data;
+	GstStaticPadTemplate *templ = list_data;
+	if (templ->direction == GST_PAD_SINK) { // Is a sink pad
+		GstCaps *capabilities = gst_static_pad_template_get_caps(templ); // Get the capabilities
+		gst_caps_foreach(capabilities, koto_playback_engine_gst_caps_iter, NULL); // Iterate over and add to mimes
+		gst_caps_unref(capabilities);
+	}
+}
+
+void koto_playback_engine_get_supported_mimetypes() {
+	// Credit for code goes to https://github.com/mopidy/mopidy/issues/812#issuecomment-75868363
+	// These are GstElementFactory
+	GList *elements = gst_element_factory_list_get_elements(GST_ELEMENT_FACTORY_TYPE_DEPAYLOADER | GST_ELEMENT_FACTORY_TYPE_DEMUXER | GST_ELEMENT_FACTORY_TYPE_PARSER | GST_ELEMENT_FACTORY_TYPE_DECODER | GST_ELEMENT_FACTORY_TYPE_MEDIA_AUDIO, GST_RANK_NONE);
+
+	GList *ele;
+	for (ele = elements; ele != NULL; ele = ele->next) { // For each of the elements
+		// GList of GstStaticPadTemplate
+		GList *static_pads = (GList*) gst_element_factory_get_static_pad_templates(ele->data); // Get the pads
+		g_list_foreach(static_pads, koto_playback_engine_gst_pad_iter, NULL);
+	}
+}