Fix build: Add pkg.m4
Play nice with themes that mix engines. Do not set any RC styles in code- move those into the gtkrc for the theme
Disable the hazy blue focus adornment because it looks silly. Cocoa seems to have stopped drawing this.
Disable the default button adornment because it is not fully implemented and sometimes looks silly
Make menus look prettier
Draw check marks as appropriate on checked menu items
Fix the checkmark drawing code for menu items and share it with draw_option
Fix the arrows on menu items that have submenus
Implement the pop up menu bluring, but leave disabled for now

Enabling it causes an extra blurred space between the right and bottom
sides of the drawn region of the menu and the border of the window shadow.
I don't know how to get rid of it :(
Fix some flickering that can occur when a menu is shown

Fixes Xamarin bug 10021
Add more themes as examples
Modernize build system
Fix drawing arrows in the menu bar