`set_primary()` and `add_primary()` had a lot of redundant code. This
unifies them, and moves them to a custom widget called `CcBatteryRow`.
This also decreases the somewhat excessive size of `cc-power-panel.c`,
and makes it easier to see the layout of widgets, now that it's
specified in xml.
Before this, `warning-battery-offset` was set to `0.03` for a "primary"
battery, and `0.05` otherwise. I expect this is a bug, so I've changed
both to `0.03`.
No other style or behavior change is intended.
