Add `linetype` parameter in `elementLine()` and `elementRect()`

[AndreiKingsley]

Add linetype for theme elements with behaviour similar to linetypes in geoms.

[alshan]

What linetype would do for elementText() ?

[AndreiKingsley]

I apologise, of course I meant elementLine().

[OLarionova-HORIS]

Added via commit: JetBrains/lets-plot@c0b386f

[alshan]

See: example notebook.