qt: Fix headers

This change is preparation for Qt 6, and it fixes an experimental build
with Qt 6.2.4.

qt: Use | instead of + for key modifiers

This change is preparation for Qt 6 where + has been deprecated, and
it fixes an experimental build with Qt 6.2.4.

qt: Update deprecated enum value

This change is preparation for Qt 6, and it fixes an experimental build
with Qt 6.2.4.
The Qt::ItemIsTristate value has been deprecated since 5.6.0 (see
ae8406d82f541f6d9112bdac192e5e4e114d56aa upstream commit).

This is a backport of core-gui#577

Depends on D18137

ninja all check-all
Test the shortcuts (e.g Ctrl + Shift + M in the overview window masks all amounts and recent transactions)