Refactor PollHandler

Refactor PollHandler to make it more reliable. This ended up with much more code than I expected as the original intent was just to stop a crash when switching rooms.
- Using a event string was flaky, changing to using an event reference is more reliable.
- Since we're only creating them from NeoChatRoom there is no need to to be able to set properties from QML so only read properties.
- Pass from the MessageEventModel rather than an invokable method.
- Create a basic test suite
- Create properties in PollHandler to remove the need to use content in PollDelegate, this means content is no longer a required role.

autotests/data/test-pollhandlerstart-sync.json

@@ -0,0 +1,38 @@
+{
+  "timeline": {
+    "events": [
+      {
+        "content": {
+            "org.matrix.msc1767.text": "test\n1. option1\n2. option 2",
+            "org.matrix.msc3381.poll.start": {
+                "answers": [
+                    {
+                        "id": "option1",
+                        "org.matrix.msc1767.text": "option1"
+                    },
+                    {
+                        "id": "option2",
+                        "org.matrix.msc1767.text": "option2"
+                    }
+                ],
+                "kind": "org.matrix.msc3381.poll.disclosed",
+                "max_selections": 1,
+                "question": {
+                    "body": "test",
+                    "msgtype": "m.text",
+                    "org.matrix.msc1767.text": "test"
+                }
+            }
+        },
+        "event_id": "$153456789:example.org",
+        "origin_server_ts": 1432735824654,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@example:example.org",
+        "type": "org.matrix.msc3381.poll.start",
+        "unsigned": {
+          "age": 1232
+        }
+      }
+    ]
+  }
+}