applet: Fix layout of the network speed graph with certain font preferences

TextMetrics uses a different text rendering backend by default, which
has led to inaccurate estimation of the width of AxisLabels. By default
TextMetrics uses QtRendering type; but our Label uses NativeRendering.
In past we used to override renderType with a conditional expression,
so considering such history, hardcoding any type seems inappropriate.
That's why we need an actual instance of Label to fetch a correct
render type at runtime.

Second part of the issue was that instead of width property the layout
should have used advanceWidth, and ceil() it, just in case. But with a
port to Label instance it doesn't matter anymore.

BUG: 461483