UI: Set loading to true when loading accessibility settings

When loading a settings pane, "loading" should be set to true to stop
signals being emitted by settings changes to trigger while the settings
are loading. This should have always been set here as well, but wasn't
detected so far because currently there are no settings in accessibility
that have signals connected that track changes.

UI/window-basic-settings-a11y.cpp

@@ -50,6 +50,7 @@ void OBSBasicSettings::LoadA11ySettings(bool presetChange)
 {
 	config_t *config = GetGlobalConfig();
 
+	loading = true;
 	if (!presetChange) {
 		preset = config_get_int(config, "Accessibility", "ColorPreset");
 
@@ -107,6 +108,8 @@ void OBSBasicSettings::LoadA11ySettings(bool presetChange)
 	}
 
 	UpdateA11yColors();
+
+	loading = false;
 }
 
 void OBSBasicSettings::SaveA11ySettings()