Implement Retroarch Importer

I'm quite rusty with Python, so please let me know if there are any (or many) grave mistakes!

data/gtk/preferences.blp

@@ -226,6 +226,20 @@ template $PreferencesWindow : Adw.PreferencesWindow {
         }
       }
 
+      Adw.ExpanderRow retroarch_expander_row {
+        title: _("Retroarch");
+        show-enable-switch: true;
+
+        Adw.ActionRow retroarch_config_action_row {
+          title: _("Install Location");
+
+          Button retroarch_config_file_chooser_button {
+            icon-name: "folder-symbolic";
+            valign: center;
+          }
+        }
+      }
+
       Adw.ExpanderRow flatpak_expander_row {
         title: _("Flatpak");
         show-enable-switch: true;

data/hu.kramo.Cartridges.gschema.xml.in

@@ -64,6 +64,12 @@
     <key name="legendary-location" type="s">
       <default>"~/.config/legendary/"</default>
     </key>
+    <key name="retroarch" type="b">
+      <default>true</default>
+    </key>
+    <key name="retroarch-location" type="s">
+      <default>"~/.var/app/org.libretro.RetroArch/config/retroarch/"</default>
+    </key>
     <key name="flatpak" type="b">
       <default>true</default>
     </key>
@@ -110,4 +116,4 @@
       <default>"[]"</default>
     </key>
   </schema>
-</schemalist>
+</schemalist>