Ensure using latest reference to Main.panel.statusArea.quickSettings

While waiting for quickSettings._brightness,
Main.panel.statusArea.quickSettings was previously be stored in a
const.  However, upon using Alt+F2 and "r" to initiate a restart of
gnome-session, this sequence was observed:

    soft-brightness-plus: Brightness slider ready, continue enable procedure
    JS ERROR: TypeError: Main.panel.statusArea.quickSettings._brightness is undefined
    _enable@file:///home/linolium/.local/share/gnome-shell/extensions/[email protected]/extension.js:348:9
    enable/this._enableTimeoutId<@file:///home/linolium/.local/share/gnome-shell/extensions/[email protected]/extension.js:320:22
    @resource:///org/gnome/shell/ui/init.js:21:20

I do not believe this actually solves the issue, but could perhaps help
in reasoning about the state of quickSettings across gnome-shell
replacement restarts.

A full solution may involve implementing retries whenever reading
_brightness, or within the first N seconds of enabling the extension.

src/extension.js

@@ -312,10 +312,9 @@ class IndicatorManager {
     enable() {
         // Subsequent 100ms checks: Wait until the _brightness object has been
         // set on quickSettings.
-        const quickSettings = Main.panel.statusArea.quickSettings;
         let tries = 0;
         this._enableTimeoutId = GLib.timeout_add(GLib.PRIORITY_DEFAULT, 100, () => {
-            if (tries >= 0 && quickSettings._brightness !== null) {
+            if (tries >= 0 && Main.panel.statusArea.quickSettings._brightness !== null) {
                 this._logger.log_debug('Brightness slider ready, continue enable procedure');
                 this._enableTimeoutId = null;
                 this._enable();