Fix left padding for "Rooms" label in the room list

This now emulates the default Kirigami heading behavior now, with the
correct amount of padding.

(cherry picked from commit be89362fdd)

src/rooms/ExploreComponent.qml

@@ -32,6 +32,8 @@ RowLayout {
 
     Kirigami.Heading {
         Layout.fillWidth: true
+        // Roughly equivalent to what Kirigami does for its built-in headings
+        Layout.leftMargin: Kirigami.Units.gridUnit - Kirigami.Units.mediumSpacing
         visible: !root.collapsed
         text: i18nc("@title", "Rooms")
     }