Make layer-visible-off more distinct from on state

The icons for the layer visible "on" and "off" states were too similar to each other, especially at small sizes. This new version of layer-visible-off addresses this by making the layers thinner which also makes it visually lighter and therefore communicates the disabled or "off" state better.

Please see the report for a screenshot that illustrates the problem https://bugs.kde.org/show_bug.cgi?id=388996

Edit: added screenshots

currently:

suggested change:

Edit 2: For completeness sake, here are two screenshots showing the different versions of the icon in the inkscape layer dialog. Please note, the "current" screenshot was taken from the bug report and therefore has a slightly different appearance.