[applets/batterymonitor] BatteryItem/ToolTip: Port to layout types

Replaces Row/Column with RowLayout/ColumnLayout, and x/y/width/height
with appropriate Layout.margins and preferrewWidth bindings.

Layout.* properties of the mainItem itself are now greatly simplified,
because top-level RowLayout manages its implicit width just fine.

Maximum width for BatteryDetails component is back, after it was lost
in 44ed25e5 refactoring.