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.