Move node actions to a menu

[felixgirault]

As of now, there is 5 action buttons per node, and there could be more in the near future (see #13, #22).
Some actions could be moved to a pop up menu, but some of them should remain accessible at all times:

• the "add child" button, as it is the most basic action in schemist
• every togglable buttons, as their state should remain obvious, namely "info", "settings" and "don't compare color" (which could disappear if the comparison system changes, see Show combinations in a grid #4)
  This would also include "fold" if is folding implemented (see Foldable nodes #22)

If all aforementioned issues were to be implemented, actions could look like:

• info
• settings
• fold
• add child
• more…
  • swap
  • remove

There would then be 5 buttons instead of 7.
Maybe the "add child" button could be placed elsewhere to get down to 4, as it is a little bit different from the others.