Draft: gcal-event-popover: Do not show an 'Edit' button in the event popover
Disable the 'Edit...' button for read-only events.
A writable flag was used to check if the event is editable following the same logic as gcal-event-editor-dialog.
But when opening the popover, the focus is set on the edit button, but when setting the button as sensitive for read-only events, the event description is selected due to the focus of the window.
Fixes #977 (closed)