Capitalization, list model and room header work

- Standardized capitalization for variables and file names everywhere in
  QML and JS, get rid of mixed camelCase/snakeCase,
  use camelCase like everywhere in Qt

- ListModel items are now stored and returned as real QObjects with
  PyQt properties and signals.
  This makes dynamic property binding a lot easier and eliminates the need
  for many hacks.

- New update(), updateOrAppendWhere() methods and roles property
  for ListModel

- RoomHeader now properly updates when the room title or topic changes

- Add Backend.pdb(), to make it easier to start the debugger from QML

harmonyqml/components/chat/Daybreak.qml

@@ -2,12 +2,13 @@ import QtQuick 2.7
 import "../base" as Base
 
 Base.HLabel {
-    text: date_time.toLocaleDateString()
     width: messageDelegate.width
-    horizontalAlignment: Text.AlignHCenter
     topPadding: messageDelegate.isFirstMessage ?
                 0 : messageDelegate.standardSpacing
     bottomPadding: messageDelegate.standardSpacing
+
+    text: dateTime.toLocaleDateString()
+    horizontalAlignment: Text.AlignHCenter
     font.pixelSize: normalSize * 1.1
     color: "darkolivegreen"
 }