Regression: Window menu not updated when using ALT+mnemonic key

Regression after the merge of !10 (merged)

To reproduce:

• Open some folder, select some files inside thefolder
• open edit menu with ALT+mnemonic key

Result: copy/cut/paste are greyed out, since menu was not updated before "show"

Proposed solution: Update menu on "show" signal, not (only) on "button-press-event"