{tool,top}bar fixes for high dpi

This moves away from measuring layout of non-typographical elements
in ems, favoring px for better consistency across devices with different
pixel densities. While that may sound backward, the reality is that CSS
pixels are device independent, and non-integer em values seem to be handled
less consistently.

Finally, there were some font size and line height tweaks made here to match
the new measurements of the top bar. This provides a consistent feel to the
top bar across devices and dpi.