gui: Add placeholder text to the sign message field

Summary: This is a backport of Core PR17186

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

In menu File > Sign Message, verify that the message text widget has a placeholder text

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8162