gui: Add close window shortcut

Summary:

CMD+W/CTRL+W is the standard shortcut to close a window without
exiting the program.

This is a backport of Core PR15768

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

Try Ctrl+W in various dialogs (e.g. in the RPC console)

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9070