libui: mnemonics in the menus

[aoloe]

currently libui does not support mnemonics for menus

• The big menu issue andlabs/libui#152
• add mnemonics to menus and menuitems andlabs/libui#105

[TomaszGasior]

libui does not support mnemonics for any element, for menus, menus elements, buttons and field labels.

[aoloe]

@TomaszGasior yes, you're right! thanks.

btw, i'm adding tickets with the label "libui" to keep track of the feature in libui i'm missing while working on this interface...