Allow elements to be anchored to the sides

[zastrowm]

Currently, they're absolutely positioned from the top-left, but the layout system needs to be able to specify the position as relative to whatever size the user specifies.

[zastrowm]

Listing out the use cases to determine what kind of solution we need to provide. Here is a list of use cases, horizontally focused, though the same use cases apply vertically.

#	Summary	Use Case
1	horizontally centered	a control that is centered horizontally within the page - as the page width changes, it remains centered
2	right anchored	a control that is anchored on the right; as the page width changes, it remains a fixed length from the right
3	left anchored	a control that is anchored on the left; as the page width changes, it remains a fixed length from the right
4	left + center anchored	a control that is anchored to the left and the center; as the page width changes, it remains a fixed length from the left and it expands its horizontal size so that the right size of the control remains on the center line

Anchors can take care of ^2 and ^3; ^1 and ^4 are more advanced use cases that might be better solved using a grid system, which we can develop later on.