Change how stage overlays & fullscreen are handled

This makes the layout behavior for stage overlays much more consistent.

Items whose overflow *should* be contained within the stage, like the
monitors and color picker loupe, are now placed within the "stage"
element directly.

Items whose overflow should *not* be contained within the stage, like
the target highlight box and dragged sprite, are now inside a "stage
overlays" container element, which is properly positioned (taking into
account the stage's border width) and sized.

This is made possible by unifying stage layout across both fullscreen &
non-fullscreen modes via moving the fullscreen overlay styles to the
StageWrapper component. Whereas previously, the stage overlays were
positioned *outside* of the stage in fullscreen and had to reposition
themselves correctly, they are now positioned the same in both
fullscreen and regular modes.