Ability to change window size programmatically

[nmsmith]

We should be able to change the size of a window programmatically, after it is created. This is useful if the window size is intended to correlate with the size of its current content, or if the window is not fully resizable but can instead be toggled between a developer-defined set of sizes (e.g. for pixel-art games).

[cmyr]

It feels to me that doing this 'correctly' requires us to be able to set the frame of the window, that is both its position and its size. Doing this should also require us to query the active displays, so that we can check how much space is available; this would tie in to #1037.