RFE: Capture: property prompt: default completion

[Phil Hudson]

I'd like us to add the ability to provide a default completion value
for a property prompt in a capture template, as already exists for a
non-property prompt.

So where at the moment we can have:

    %^{prompt|default|completion2|completion3|...}

I want, by analogy:

    %^{prop|default}p

with the remaining completions provided by the #+prop_ALL in-buffer
setting, and with the implied constraint that "default" is a member of
that set.