Make canvas use a one-time Timer to update

CURA-4182

The canvas approach has a drawback on the UI update: when a printcore
or a material is changed, calling requestPaint() to the canvas doesn't
trigger an immediate update. The solution is to have a built-in Timer in
the Canvas and to use that to update. Somehow this just works. The
problem with this approach is that the Timer constantly updates the
canvas which eats a lot of CPU power. To solve this, I changed the timer
to be a one-time timer and it will be started when there is a
material/print core update.

resources/qml/SidebarSimple.qml

@@ -143,6 +143,7 @@ Item
                         }
                     }
                     resolutionSlider.currentQualityIndex = undefined;
+                    backgroundBarUpdateTimer.start();
                 }
 
                 function updateBar()
@@ -246,9 +247,10 @@ Item
                     anchors.fill: parent
 
                     Timer {
-                        interval: 16
+                        id: backgroundBarUpdateTimer
-                        running: true
+                        interval: 10
-                        repeat: true
+                        running: false
+                        repeat: false
                         onTriggered: backgroundBar.requestPaint()
                     }