WIP: Grid bag layout

[Trashtalk217]

Here's my first draft for a grid layout where an element can stretch over multiple gridcells. I'm planning to add the features the grid-layout has, so the grid bag layout can perhaps replace the grid layout. There are a lot of optimizations still left to do, but most of them are doable. The major ones I'm still unsure about are the following two,

• Implementing stretch
• Getting an element based on it's position in the grid to not be O(n)

There are probably some other things I'm going to run into, but this is it, for now.