Merge #18101: qt: Fix deprecated QCharRef usage

ac57859e53167f4ff3da467b616b0902c93701a9 qt: Fix deprecated QCharRef usage (Hennadii Stepanov)

Pull request description:

  From Qt docs:
  - [`QKeyEvent::text()`](https://doc.qt.io/qt-5/qkeyevent.html#text):
  > Return values when modifier keys such as Shift, Control, Alt, and Meta are pressed differ among platforms and could return an empty string.

  - [`QString::operator[]()`](https://doc.qt.io/qt-5/qstring.html#operator-5b-5d):

  > **Note:** Before Qt 5.14 it was possible to use this operator to access a character at an out-of-bounds position in the string, and then assign to such a position, causing the string to be automatically resized. Furthermore, assigning a value to the returned `QCharRef` would cause a detach of the string, even if the string has been copied in the meanwhile (and the `QCharRef` kept alive while the copy was taken). These behaviors are deprecated, and will be changed in a future version of Qt.

  Since Qt 5.14 this causes a `QCharRef` warning if any modifier key is pressed while the splashscreen is still displayed.

  Fix #18080.

  Note: Ctrl+Q will also close the spashscreen now.

ACKs for top commit:
  jonasschnelli:
    utACK ac57859e53167f4ff3da467b616b0902c93701a9

Tree-SHA512: a7e5559410bd05c406007ab0243f458b82d434b0543276ed331254c8d7a6b1aaa54d0b406f799b830859294975004380160f8af04ba403d3bf185d51e6784f54

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