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/CMakeLists.txt

@@ -64,3 +64,9 @@ ecm_add_test(
     LINK_LIBRARIES neochat Qt::Test
     TEST_NAME windowcontrollertest
 )
+
+ecm_add_test(
+    pollhandlertest.cpp
+    LINK_LIBRARIES neochat Qt::Test
+    TEST_NAME pollhandlertest
+)