Implementing rollup diplomacy ribbon banners

[chaorace]

This PR adds a method to hide individual diplomacy banners. It achieves this by having the banners able to be toggled from a hidden state to an active one, while also being able to "peek" the banner by mousing over the corresponding portrait while hidden

Currently, the banners start hidden by default and unroll when you mouse
over the diplomacy ribbon portraits. Clicking the banners stickies them
open.

TODO:

• Better indication that the banners are clickable (some kind of
  highlighting on mouseover?)
• An option to invert the default state to opened (I plan to make this
  option default to true, to match the current behavior of the diplomacy
  ribbon banners)
• Persisting the states of the banners on save/reload

[ricanuck]

The banner is partially visible in the case of the local player due to the different vertical alignment of the local player leader icon on the diplo ribbon with respect to the other leaders. Not sure how to solve it.
Also, there seems to be a slight misalignment of the icons with respect to the ribbon, I guess that is due to your trick of rendering the icons outside the ribbon?

[chaorace]

@ricanuck We could always, you know, have it go up further

[chaorace]

@ricanuck We could always just have the banners animate higher up. As for the portrait appearing less aligned than before, I didn't notice anything like that during testing. It's entirely possible I missed it, and I can't test it right now, but I do know there were always minor alignment issues and that perhaps it's only become obvious now under scrutiny. Either way, I'll look into possibly improving that later today