Various fixups

The biggest change is in my first commit, which adds everything in src/Atlas/Plugins to git. Otherwise the build is broken, and you have to copy everything from src/Assembly/Plugins. Not sure why Assembly is preserved on this branch, anyhow.

Other changes:
 - Set AllowsTransparency to False, so the window uses accelerated graphics instead of software rendering
 - Removed DwmDropshadow and the dwmapi.dll dependency (interfered with AllowsTransparency=False)
 - Switched to native resizing for performance and compatibility with aero snap (Win+arrows, dragging to screen edge works)
 - Removed some redundant grids
 - Enlarged the draggable area at the top of the window (no longer obscured by the menu)
 - Fixed the maximize button hitbox