qt: Avoid potential -Wdeprecated-enum-enum-conversion warnings

Summary:
Use human-readable strings in preference to hard-coded integers

This is recommended by Qt docs.
See: https://doc.qt.io/qt-5/qkeysequence.html#details

Also this change avoids -Wdeprecated-enum-enum-conversion warnings.

Backport note: these deprecations become errors with Qt6

This is a backport of core#24624

Test Plan:
ninja all check-all

Run bitcoin-qt, check a few of the keyboard shortcuts (open network dialog, minimize window...)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18137