Mobile--specific theme to follow the design

Add a plasma-mobile specific theme, which follows more the mockups, which for now only provides widgets/background. it has a bigger round edges and no transparency. (hironically is derived from the plasma 5.0 version i designed back then). even though the desktop style should probably go in a similar direction, having special cases here probably makes sense

this should avoid most of the need of manual rectangles with shadows instead.