Properly specify icon name

This way we don't have material-specific names in KAlgebra

mobile/plugins/Plot2D.qml

@@ -46,7 +46,7 @@ KAlgebraPage
                             }
                         }
                         Button {
-                            iconName: "content/add"
+                            iconName: "list-add"
                             onClicked: {
                                 input.selectAll()
                                 view.addFunction(input.text, app.variables)

mobile/plugins/Plot3D.qml

@@ -58,7 +58,7 @@ KAlgebraPage
                         }
                     }
                     Button {
-                        iconName: "content/add"
+                        iconName: "list-add"
                         onClicked: {
                             input.selectAll()
                             var err = view.addFunction(input.text, app.variables)

mobile/plugins/widgets/material/Button.qml

@@ -23,7 +23,7 @@ Loader
         IconButton {
             Layout.minimumWidth: implicitWidth*2
             Layout.minimumHeight: implicitHeight*2
-            iconName: root.iconName
+            iconName: rootItem.iconConvert[root.iconName]
         }
     }
 

mobile/plugins/widgets/material/KAlgebraMobile.qml

@@ -25,7 +25,8 @@ ApplicationWindow
         "adjustrgb": "action/view_carousel",
         "kspread": "editor/format_list_numbered",
         "document-properties": "content/content_copy",
-        "help-about": "action/help"
+        "help-about": "action/help",
+        "list-add": "content/add"
     }
     
     initialPage: Page {