Handle room invites

Add them to the roomList, and have separate sections:
Invites and Rooms.

harmonyqml/components/sidePane/RoomCategoryDelegate.qml

@@ -0,0 +1,16 @@
+import QtQuick 2.7
+import "../base" as Base
+
+Base.HLabel {
+    width: roomList.width
+    height: text.height
+
+    // topPadding is provided by the roomList spacing
+    bottomPadding: roomList.spacing
+
+    text: section
+    elide: Text.ElideRight
+    maximumLineCount: 1
+
+    font.bold: true
+}

harmonyqml/components/sidePane/RoomList.qml

@@ -18,4 +18,7 @@ ListView {
     spacing: 8
     model: Backend.models.rooms.get(forUserId)
     delegate: RoomDelegate {}
+
+    section.property: "category"
+    section.delegate: RoomCategoryDelegate {}
 }