Make every applet its own focus scope

Summary:
Like Loaders an applet should be a focus scope. It allows an applet to
set an item to have focus which will then have activeFocus when that
applet gets focus.

Also makes it easy to test if any item within the plasmoid has
activeFocus.

Test Plan:
Keyboard nav in kicker/klipper still work
Added opacity: Plasmoid.activeFocus ? 1 : 0 in an applet and it worked
as expected

Reviewers: #plasma

Subscribers: plasma-devel, #frameworks

Tags: #plasma, #frameworks

Differential Revision: https://phabricator.kde.org/D6152

src/plasmaquick/appletquickitem.cpp

@@ -403,6 +403,7 @@ AppletQuickItem::AppletQuickItem(Plasma::Applet *applet, QQuickItem *parent)
     : QQuickItem(parent),
       d(new AppletQuickItemPrivate(applet, this))
 {
+    setFlags(QQuickItem::ItemIsFocusScope);
     d->init();
 
     if (d->applet) {