AppMenu missing mnemonics (Kirigami MnemonicAttached conflict)

Global AppMenu applet doesn't show mnemonics underlines. I've tracked this bug down to some logic override by Kirigami's MnemonicAttached class.

Kirigami's MnemonicAttached removes all of acceleration marks while Alt is not pressed in current window. AppMenu has some similar logic but because then it's being processed by MnemonicAttached even when Alt is pressed underlines are being trimmed. It works fine for plasmawindowed when it's in focus but not as a panel applet.

I see a possible solution is either:

• to add an ability to force mnemonics showing in MnemonicAttached (and ToolButton then as a separate property)
• or maybe somehow check for global Alt-press in Kirigami (maybe as a new ControlType logic but it's used for priorities so I don't know).

But I'm not sure how clean it is and will be glad to implement any good enough solution.

@davidedmundson