Add ActionMenu to pebblejs

[youtux]

Hello Meiguro, it would be great to have the ActionMenu available in pebblejs. I started tinkering with pebblejs to add this functionality, but I'm not sure how you would like to be implemented. I wanted it to be available for both SimplyWindows and SimplyMenus, so I thought to add

struct SimplyActionMenu *action_menu;

to the Simply struct, and start from there.

Do you have any guideline, so I can help with this?