Display the canvas label

[ggeisler]

Part of #2110.

Goals of adding the canvas label are:

• Add a visible canvas label to enable the user to see the label of the current canvas without having to open the sidebar
• Provide a secondary, unobtrusive way of indicating when a given window has focus

See the B callout in the mockup below:

Canvas label styling

• Centered on the canvas, below the canvas nav buttons. In the mockups below I placed the canvas label 3px up from the bottom of the canvas.
• Font size: use the caption class as documented in Typography classes & styles #2093; color: same as color of the canvas nav buttons (I think the color might be adjusted soon with updates to the color theme, but this is good enough for now).
• Labels longer than can fit in the current canvas width should be truncated so the label always uses one line only.