Increase Qt requirement to 5.12 and remove pre-5.12 workarounds

app/gui/CenteredGridView.qml

@@ -2,10 +2,6 @@ import QtQuick 2.9
 import QtQuick.Controls 2.2
 
 GridView {
-    // Detect Qt 5.11 or earlier using presence of synchronousDrag.
-    // Prior to 5.12, the leftMargin and rightMargin values did not work.
-    property bool hasBrokenMargins: this.synchronousDrag === undefined
-
     property int minMargin: 10
     property real availableWidth: (parent.width - 2 * minMargin)
     property int itemsPerRow: availableWidth / cellWidth
@@ -15,11 +11,6 @@ GridView {
     function updateMargins() {
         leftMargin = horizontalMargin
         rightMargin = horizontalMargin
-
-        if (hasBrokenMargins) {
-            anchors.leftMargin = leftMargin
-            anchors.rightMargin = rightMargin
-        }
     }
 
     onHorizontalMarginChanged: {
@@ -27,15 +18,6 @@ GridView {
     }
 
     Component.onCompleted: {
-        if (hasBrokenMargins) {
-            // This will cause an anchor conflict with the parent StackView
-            // which breaks animation, but otherwise the grid will not be
-            // centered in the window.
-            anchors.fill = parent
-            anchors.topMargin = topMargin
-            anchors.bottomMargin = bottomMargin
-        }
-
         updateMargins()
     }