BUI - LayoutManager and much more

Last weekend I progressed on restructuring the code. I managed to get rid of some cruft. Of course there is some new cruft to fix. :)

Here are the major changes made summarized:

• Implemented LayoutManager to handle calculating all layout related things for all user interface containers/elements. This made it possible to clean up the mess caused by different coordinate systems used by the backend and OpenGL based frontends. In BUI origin of a window is imagined to be at the top left corner. Also the y axis is inverted (value of y is inverse of y in Cartesian coordinate system).
• Cleaned up the way render is defined in AbstractObject. This leads cleaner frontend code as coordinate doesn't have to be passed on anymore. Also super is no longer needed.
• Implemented Separator element. It just draws a line based on the container inside which it has been placed. Note that name attribute works only in case it has been placed inside VerticalContainer (vertical flow leads to horizontal separator and vice versa). In this case the name is rendered in the middle of the separator. Also line is not drawn on that specific location. To implement the missing case I probably have to look into Pango and use Cairo to render Pango text into an OpenGL surface or just a raw image which isn't as nice.
• I also began to separate drawing code into it's own modules. In case of OpenGL the idea is to make the code generic so that the actual import can be handled on the frontend side. This means the same functions can be reused on multiple frontends.
• Converted Fill to element. It did not make sense to define Fill as a container as it logically cannot contain other elements. Therefore it is an element.
• Cleaned up the way Blender WindowManager calculates mouse coordinates. It should work alright now. There was a tiny glitch before.

There are still plenty of things to implement before the first official release but at least it's progressing. :) In the next weeks I hope to implement features more visible to the user (CustomContainer with free layout (element coordinates as relative?), event mapper, scrollbars, auto width/height, StyleManager?, ???).