Timeline: Avoid clip on TimeDisplay

The only thing that needs clipping are the thin vertical lines. We can
manually manage those with the "visible" property. Clipping with the
"clip" property on the parent element can cause costly rebatching for
the whole content while scrolling.

Change-Id: Ia31eeefc4c0a943b00ea47b12c5c9d49b0f004e3
Task-number: QTCREATORBUG-14983
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/libs/timeline/qml/MainView.qml

@@ -172,7 +172,6 @@ Rectangle {
         windowStart: zoomControl.windowStart
         rangeDuration: zoomControl.rangeDuration
         contentX: content.contentX
-        clip: true
     }
 
     ButtonsBar {

src/libs/timeline/qml/TimeDisplay.qml

@@ -151,6 +151,7 @@ Item {
                             width: timeDisplay.pixelsPerSection
 
                             Rectangle {
+                                visible: column.stableIndex !== 0 || (-row.x < parent.x + x)
                                 color: "#CCCCCC"
                                 width: 1
                                 anchors.top: parent.top