GUI: Change the receive button to respond to keypool state changing

Summary:

• Check for more than private keys disabled to show receive button

• Notify the GUI that the keypool has changed to set the receive button

Whenever the keypool changes (new keys generated, new seed set,
keypool runs out, etc.), notify the GUI that the keypool has changed. The
receive button can then be enabled and disabled as necessary.

This is a backport of Core PR15225

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4840