Add extruder button bar that shows icons for each extruder in a horizontal row 💪

Make ExtruderButton more generic by moving functionality out.

Remove background color from ToolbarButton and add that background color to the Toolbar.qml.

Fix Toolbar border being created by overlapping rectangles instead of using border

Add scaling options for ToolBarButtons icon

CURA-9793

resources/qml/Widgets/ExtruderSelectorBar.qml

@@ -0,0 +1,40 @@
+import QtQuick 2.12
+import QtQuick.Controls 2.12
+import QtQuick.Layouts 1.3
+
+import UM 1.5 as UM
+import Cura 1.5 as Cura
+
+Row
+{
+    id: extruderSelectionBar
+    property alias model: extruderButtonRepeater.model
+
+    spacing: 0
+    width: parent.width
+    height: childrenRect.height
+
+    Repeater
+    {
+        id: extruderButtonRepeater
+
+        delegate: Item
+        {
+            width: {
+                const maximum_width = Math.floor(extruderSelectionBar.width / extruderButtonRepeater.count);
+                return Math.min(UM.Theme.getSize("large_button").width, maximum_width);
+            }
+
+            height: childrenRect.height
+
+            Cura.ExtruderButton
+            {
+                extruder: model
+                isTopElement: extrudersModel.getItem(0).id == model.id
+                isBottomElement: extrudersModel.getItem(extrudersModel.rowCount() - 1).id == model.id
+                iconScale: 0.6
+                buttonSize: UM.Theme.getSize("large_button").width
+            }
+        }
+    }
+}