Consider factoring out "bitcoins" string in AskPassphraseDialog

[JeremyRand]

The Bitcoin-Qt GUI contains the word "bitcoins" in two strings in AskPassphraseDialog. In the same spirit as bitcoin/bitcoin#7192 , it would be nice to factor that out into a single constant string, so that translations are more consistent, and forks that call their coins something other than "bitcoins" can decrease their diff size relative to upstream. Seems like maybe the BitcoinUnits class could be used for this, since it already defines the string "Bitcoins".

[fanquake]

If you want to make changes that improve the code here, that's fine, but

and forks that call their coins something other than "bitcoins" can decrease their diff size relative to upstream.

that alone is not going to be a reason for us to make changes in our codebase.

[JeremyRand]

If you want to make changes that improve the code here, that's fine, but

and forks that call their coins something other than "bitcoins" can decrease their diff size relative to upstream.

that alone is not going to be a reason for us to make changes in our codebase.

Yes, I know. The primary reason I'd hope for this change to be made is just that it makes the code cleaner, as with the merged PR that I linked.