qt: make receive coins tab look more consistent with send coins tab

Use a GridLayout instead of a FormLayout. This makes the alignment
of the labels consistent between the tabs.
Generalize the remove-outputs check for fully-prunable transactions.

Instead of explicitly testing for the presence of any output, and
dealing with this case specially, just interpret it as an empty
CCoins.

The case previously caught using the HaveCoins check, is now handled
by the generic outs != outsBlock test.
Merge pull request #3163 from sipa/allunspendable

Generalize the remove-outputs check for fully-prunable transactions.