Allow finding out more information about commits

When in the day view, allow double-clicking on a data point to find out
what changed with the commit that the measurement corresponds to - a pane
is opened at the bottom of the screen. Add keybindings for navigating to
adjacent points.
Add hints box in the upper right of the screen

The double-clicking and key shortcuts might be a bit hard to find out - go with
the sledge-hammer approach which shows context-sensitive hints.
Display all preceding commit with the details for a build

Instead of just showing the commits that directly caused the build,
show the changes since the previous successful test run for the target.