fix #301,enable only one,previews,highlight

-- there is a conflict between preview windows
and highlight effect, it creates a crash when both
are activated during hovering. So we enable in each case
only one of both

plasmoid/contents/ui/main.qml

@@ -451,7 +451,10 @@ Item {
 
         taskManagerItem: root
         toolTipItem: toolTipDelegate
-        highlightWindows:root.highlightWindows
+        //! there is a conflict between preview windows and highlight effect, it creates
+        //! a crash when both are activated during hovering. So we enable in each case
+        //! only one of both
+        highlightWindows: root.highlightWindows && !root.showPreviews
 
         onAddLauncher: {
             tasksModel.requestAddLauncher(url);

shell/contents/configuration/TasksConfig.qml

@@ -108,6 +108,7 @@ PlasmaComponents.Page {
                 Layout.leftMargin: units.smallSpacing * 2
                 text: i18n("Preview windows on hovering")
                 checked: plasmoid.configuration.showToolTips
+                enabled: !highlightWindowsChk.checked
 
                 onClicked: {
                     plasmoid.configuration.showToolTips = checked
@@ -119,6 +120,7 @@ PlasmaComponents.Page {
                 Layout.leftMargin: units.smallSpacing * 2
                 text: i18n("Highlight windows on hovering")
                 checked: plasmoid.configuration.highlightWindows
+                enabled: !showPreviewsChk.checked
 
                 onClicked: {
                     plasmoid.configuration.highlightWindows = checked