Add a button in Settings that opens the config folder

Shows the folder with config.ini, the saved settings and the cached
characters and songs in the system's file manager.

Closes #45

popups/SettingsPopup.tscn

@@ -158,6 +158,21 @@ focus_mode = 0
 action_mode = 0
 text = "Count plays in statistics"
 
+[node name="ConfigFolderRow" type="HBoxContainer" parent="SettingsPanel/VBoxContainer"]
+layout_mode = 2
+
+[node name="Title" type="Label" parent="SettingsPanel/VBoxContainer/ConfigFolderRow"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_type_variation = &"SettingsRowLabel"
+text = "Settings and cached files"
+
+[node name="OpenConfigFolderButton" type="Button" parent="SettingsPanel/VBoxContainer/ConfigFolderRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Open folder"
+
 [node name="FolderDialog" type="FileDialog" parent="."]
 unique_name_in_owner = true
 title = "Choose a music folder"