Implement image substitution

[marpontes]

From the edit dialog, the use should see a list of images that are used on his template and he should be able to upload his own images to replace each of them.

I think of it like a list of images, placed one below the other and one action button like [Upload]. By clicking that button, he'll be presented a file dialog so he could choose the image to replace that one.

We could list images from assets/img folder and enable the replacement of those ones from the interface. That's a limited feature, once it forces the user to place his images there and it's also file extension limiting, but more advanced users can still make templates with inline css and put Pebble variables in there to tackle this issue.