complete basic sketch editing, boolean, extrusion

mesh fixes, start sketch module
webkit icon fix

add edge to face editing in mesh:tool
add cross-links between kiri <> mesh for electron app
a few low level bugs squashed plus css tweaks

add new CSG primitives. update to mesh:tool

fix some non-manifold slicing cases
fix on-belt layer detection, stringing with brims

bug fixes #277, #280
add collapsible settings groups
fix depth first with multiple pocket selections
electron apps detect when out of date
update help menu

add arbitrary slice angles for belt fdm devices
bug fixes and code comments

Bug fixes, polygon code cleanup, poly arc detection for (future) drill op update

added Gerber file import. bug fixes

bug fixes