Menu: fix key navigation on Windows

Do not force Qt::PopupFocusReason but let QGuiApp
processActivatedEvent() check the window flags and
do the right thing.

Task-number: QTBUG-41935
Change-Id: Ic97c1d3a7f42a67e43d34b0c035c10d1e26412d7
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Base style: stylize mnemonics

Change-Id: I5846688e5e93c35925b735f62518ae56bf0e57e2
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
AbstractCheckable: handle mnemonic

Change-Id: I402d674cb5a88fb0bc7bd41062c97703bc8aefbc
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Android style: hide mnemonics

Change-Id: I9a29e3c10ceb9854aff72bdf0b8399e475abdf90
Task-number: QTBUG-41976
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
BasicButton: handle mnemonics

Change-Id: Ie5cb9d5e4d5d48aedf211f4dff1f6a44cc1c9571
Reviewed-by: Liang Qi <liang.qi@digia.com>