eg. LV2 externalUI windows are not managed by ardour; Ardour cannot
intercept mapped() or delete_event() signals and hence also not
create a visibility-tracker.
First call to WindowProxy::toggle() creates the window, and the 2nd call
crashed since 6ca8ec5141, due to missing vistracker.
e.g. externalUI plugin windows, the processor-box uses WindowProxy
for all plugin-UIs incl. externalUIs.
58db958839