Fix missing reference projector in Timer/Message dialog

[Elblinator]

Current behaviour: If you open the "select projection?" dialog for a timer or message then every projector but the reference projector is selectable

Reproduction: In a meeting:

1. Navigate to a projector-list
2. Have at least two projectors
3. Navigate into any projector-detail-view
4. Open the "select projection?"-dialog for a timer/message
5. The reference projector is not selectable

Wanted behaviour: The reference projector should be selectable

[reiterl]

Where can I open a "select projection?" dialog? I didn't find it.

[Elblinator]

used Icon 'open_in_new'

The title from the dialog is 'Select projection?'