Hide orphans if PoS minting

[shane-kerr]

This actually comes from here:

mintcoinproject/mintcoin#5

GitHub user I3ck suggests:

PoS orphans should be hidden in the transaction history.
The community seems to assume to have lost coins there, which is simply not the case.
Hiding PoS orphans e.g. after half an hour without confirmations would make the history look way less 'bugged'

I basically agree with this proposal (although it can mask problems). I propose that we will want a checkbox on the "Transactions" pane that says something like "☐ Show Orphans" or "☒ Show Orphans" with a tooltip explaining (briefly) what an orphan is.

We may also want to warn the user (possibly via one of the icons in the bottom right of the wallet) if more than some amount - maybe 5 or 10? - minting attempts in a row have failed.