qt: Fix deprecated QCharRef usage

Summary:
This change prevent a QCharRef deprecation warning if any modifier key is pressed while the spashscreen is still displayed, for Qt >= 5.14.
As a side effect, Ctrl+Q and Shift+Q will also close the splashscreen now.

This is a backport of core PR18101

Test Plan:
ninja && src/qt/bitcoin-qt -regtest
Press Q, or Ctrl+Q or Shift+Q while on the splashscreen.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8741