[backport#16822 1/3] gui: rename encrypt(), blank(), disablePrivateKeys()

Summary:
to be consistent in naming.

Backport of Core PR16822 - part 1 of 3
Commit https://github.com/bitcoin/bitcoin/pull/16822/commits/43aa9b0d790840e30c1cd128e67946ffcbfeb721

Note: on our side the spelling of the method name askPassphrase was already good
when it was first backported (see D7105)

Test Plan:
Method renaming without change in the logic.
Just compile to make sure it didn't break anything.
ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8006