make the sizing of the toolbuttons more coherent

Summary:
this makes the toolbutton icons follow a standard size.
Maybe one could make it follow the icon group size props? like icon.width and icon.height  if those props are set.

Before patch:

{F7274098}

After patch:

{F7274097}

Reviewers: #plasma, mart, ngraham

Reviewed By: ngraham

Subscribers: apol, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D23367

src/declarativeimports/plasmacomponents3/ToolButton.qml

@@ -45,14 +45,23 @@ T.ToolButton {
     flat: true
 
     contentItem: RowLayout {
+
+        Item {
+            Layout.preferredWidth: Kirigami.Units.iconSizes.smallMedium + Kirigami.Units.largeSpacing
+            Layout.fillWidth: !label.visible
+            visible: icon.source.length > 0
             PlasmaCore.IconItem {
-            Layout.fillWidth: true
-            Layout.fillHeight: true
+                id: icon
+                anchors.centerIn: parent
+                height: Kirigami.Units.iconSizes.smallMedium
+                width:  Kirigami.Units.iconSizes.smallMedium
                 colorGroup: PlasmaCore.ColorScope.colorGroup
-            visible: source.length > 0
                 source: control.icon ? (control.icon.name || control.icon.source) : ""
             }
+        }
+
         Label {
+            id: label
             Layout.fillWidth: true
             visible: text.length > 0
             text: control.Kirigami.MnemonicData.richTextLabel