[applets/batterymonitor] Fix it, pt.5: Port BatteryItem to layout types

Some flattening and layout enhancements. There's not much to show for it, before and after are supposed to look the same. Except the ToolTip, which got fixed width instead of expanding to whatever width it liked.

I recommend testing with these patches:

138,138
-        active: !detailsLoader.active
+        active: true

163,163
-                visible: !root.isBroken
+             // visible: !root.isBroken

Before:

After: