reintroduce containmentForScreen with defaultPlugin

reintroduces an api call from plasma1:
its the only way to solve
https://bugs.kde.org/show_bug.cgi?id=337200
basically to avoid a crash when plasma starts with missing containments in the appletsrc and a locked corona, or a screen added with locked widgets.

it's the only entry point that allows a creation of a containment when widgets are locked
REVIEW:119513
CCBUG:337200

src/plasma/corona.cpp

@@ -182,6 +182,26 @@ Containment *Corona::containmentForScreen(int screen) const
     return 0;
 }
 
+Containment *Corona::containmentForScreen(int screen,
+                                          const QString &defaultPluginIfNonExistent, const QVariantList &defaultArgs)
+{
+    Containment *containment = containmentForScreen(screen);
+    if (!containment && !defaultPluginIfNonExistent.isEmpty()) {
+        // screen requests are allowed to bypass immutability
+        if (screen >= 0 && screen < numScreens()) {
+            Plasma::Types::ImmutabilityType imm = d->immutability;
+            d->immutability = Types::Mutable;
+            containment = d->addContainment(defaultPluginIfNonExistent, defaultArgs, 0, false);
+            if (containment) {
+               // containment->setScreen(screen);
+            }
+            d->immutability = imm;
+        }
+    }
+
+    return containment;
+}
+
 QList<Containment *> Corona::containments() const
 {
     return d->containments;

src/plasma/corona.h

@@ -107,6 +107,19 @@ public:
      */
     Containment *containmentForScreen(int screen) const;
 
+    /**
+     * Returns the Containment for a given physical screen and desktop, creating one
+     * if none exists
+     *
+     * @param screen number of the physical screen to locate
+     * @param defaultPluginIfNonExistent the plugin to load by default; "null" is an empty
+     * Containment and "default" creates the default plugin
+     * @param defaultArgs optional arguments to pass in when creating a Containment if needed
+     */
+    Containment *containmentForScreen(int screen,
+                                      const QString &defaultPluginIfNonExistent,
+                                      const QVariantList &defaultArgs = QVariantList());
+
     /**
      * Returns the number of screens available to plasma.
      * Subclasses should override this method as the default