Add visual accenting for the 'Create new receiving address' button

Summary:

qt: Make "Create new receiving address" default unconditionally

This change makes this button visually accented, and gives to users a
hint about the primary action.
This commit does not change behavior.

qt: Leverage the default "Create new receiving address" button

When Enter or Return is pressed the default button will be always
clicked. All buttons can always be clicked from the keyboard by pressing
spacebar when the button has focus.

This is basically a simplification of the way pressing return in the
"Receive" tab of bitcoin-qt submits the form, doing it the standard Qt
way without additional code.

This is a backport of core-gui#39

Test Plan:
ninja && src/qt/bitcoin-qt

In the Receive tab of the main window, verify that pressing Return still
submits the form and creates a new address.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10135