framework part of the applet alternatives chooser

this is the little part in plasma-framework for the applet alternatives chooser.
works together the branch mart/alternativesConfig of plasma-workspace and plsma-desktop.
for how it looks and why, see the vdg forum thread:
https://forum.kde.org/viewtopic.php?f=285&t=122067&p=315919#p315919

still possible problems:

I'm not sure about using a new desktop file entry X-Plasma-Provides, maybe Categories could be enough, but it may produce many false positives as well

REVIEW:119409

src/plasma/applet.cpp

@@ -494,6 +494,12 @@ void Applet::flushPendingConstraintsEvents()
             action->setEnabled(unlocked);
         }
 
+        action = d->actions->action("alternatives");
+        if (action) {
+            action->setVisible(unlocked);
+            action->setEnabled(unlocked);
+        }
+
         action = d->actions->action("configure");
         if (action && d->hasConfigurationInterface) {
             bool canConfig = unlocked || KAuthorized::authorize("plasma/allow_configure_when_locked");

src/plasma/containment.h

@@ -235,6 +235,11 @@ Q_SIGNALS:
      */
     void configureRequested(Plasma::Applet *applet);
 
+    /**
+     * Emitted when the user wants to chose an alternative for this applet or containment.
+     */
+    void appletAlternativesRequested(Plasma::Applet *applet);
+
     /**
      * Emitted when the wallpaper plugin is changed
      */

src/plasma/data/servicetypes/plasma-applet.desktop

@@ -49,3 +49,6 @@ Type=bool
 
 [PropertyDef::X-Plasma-DBusActivationService]
 Type=QString
+
+[PropertyDef::X-Plasma-Provides]
+Type=QStringList

src/plasma/private/applet_p.cpp

@@ -34,6 +34,7 @@
 #include <kkeysequencewidget.h>
 #include <kglobalaccel.h>
 #include <KConfigLoader>
+#include <KServiceTypeTrader>
 
 #include "containment.h"
 #include "corona.h"
@@ -157,6 +158,32 @@ void AppletPrivate::init(const QString &packagePath, const QVariantList &args)
                   "Could not create a %1 ScriptEngine for the %2 widget.",
                   api, appletDescription.name()));
     }
+
+    if (!q->isContainment() && q->pluginInfo().isValid()) {
+        QString constraint;
+        QStringList provides = q->pluginInfo().property("X-Plasma-Provides").value<QStringList>();
+        if (!provides.isEmpty()) {
+            bool first = true;
+            foreach (const QString &prov, provides) {
+                if (!first) {
+                    constraint += " or ";
+                    first = false;
+                }
+                constraint += "'" + prov + "' in [X-Plasma-Provides]";
+            }
+
+            KPluginInfo::List applets = KPluginInfo::fromServices(KServiceTypeTrader::self()->query("Plasma/Applet", constraint));
+            if (applets.count() > 1) {
+                QAction *a = new QAction(QIcon::fromTheme("preferences-desktop-default-applications"), i18n("Alternatives..."), q);
+                q->actions()->addAction("alternatives", a);
+                QObject::connect(a, &QAction::triggered,[=] {
+                    if (q->containment()) {
+                        emit q->containment()->appletAlternativesRequested(q);
+                    }
+                });
+            }
+        }
+    }
 }
 
 void AppletPrivate::cleanUpAndDelete()

src/scriptengines/qml/plasmoid/containmentinterface.cpp

@@ -751,6 +751,10 @@ void ContainmentInterface::addAppletActions(QMenu &desktopMenu, Plasma::Applet *
         if (configureApplet && configureApplet->isEnabled()) {
             desktopMenu.addAction(configureApplet);
         }
+        QAction *appletAlternatives = applet->actions()->action("alternatives");
+        if (appletAlternatives && appletAlternatives->isEnabled()) {
+            desktopMenu.addAction(appletAlternatives);
+        }
 
         QAction *runAssociatedApplication = applet->actions()->action("run associated application");
         if (runAssociatedApplication && runAssociatedApplication->isEnabled()) {