Fixed GStreamer error printing and added option to disable sending cover

art to Discord RPC (Security issue)

GStreamer's GError is a struct, not a char*, with GError->message being
the actual char* message.

Since Subsonic/OpenSubsonic servers don't have a way of sending cover
art unauthenticated, sending cover art to Discord using Discord RPC
exposes an authenticated URL to Discord servers (But only Discord since
it is then proxy'd). Added a config option to disable this behavior and
only use the app icon if the user doesn't feel comfortable enough to
trust Discord (understandable tbh)

src/configHandler.c

@@ -47,6 +47,7 @@ int configHandler_Read(configHandler_config_t** configObj) {
     (*configObj)->discordrpc_enable = false;
     (*configObj)->discordrpc_method = 0;
     (*configObj)->discordrpc_showSysDetails = false;
+    (*configObj)->discordrpc_showCoverArt = false;
     (*configObj)->audio_equalizer_enable = false;
     (*configObj)->audio_equalizer_followPitch = false;
     (*configObj)->audio_equalizer_graphCount = 0;
@@ -251,6 +252,13 @@ int configHandler_Read(configHandler_config_t** configObj) {
         }
     }
 
+    cJSON* discordrpc_showCoverArt = cJSON_GetObjectItemCaseSensitive(discordrpc_root, "showCoverArt");
+    if (cJSON_IsBool(discordrpc_showCoverArt)) {
+        if (cJSON_IsTrue(discordrpc_showCoverArt)) {
+            (*configObj)->discordrpc_showCoverArt = true;
+        }
+    }
+
     // Make an object from audio
     cJSON* audio_root = cJSON_GetObjectItemCaseSensitive(root, "audio");
     if (audio_root == NULL) {