Microgame general UI

[paulloz]

We want to have a consistent UI across microgames. It should be self-contained in its own PackedScene and contain everything common to all microgames: timer, instructions, layout, etc. The root of this scene should probably be a CanvasLayer. It should also be responsible for initializing these components.

This shouldn't be serialized into the microgame template scene itself. We want to instantiate it in the same way we currently instantiate the timer scene here. Take in consideration that #46 is currently open and works on the same components. See how the initialization of the instructions' text is done in that PR.

Eventually, it'll become possible to change how this UI looks with themes. But this shouldn't be the focus at the moment.

[DrSlowpokePhd]

Here's a simple layout for the common microgame UI I made in Figma https://www.figma.com/file/vatDiyX5tjEwaIClNgRGgb/CommonUI?node-id=0%3A1&t=4Zp2smWhVlYTkJn2-1.