Removed FLTK dependencies from headless stage

[dkargin]

I've made some work to separate simulation part from FLTK.

I've separated libstage to two separate libraries: libstage-core and libstage. libstage-core implements headless simulation, and now it contains no references to FLTK. libstage includes libstage-core and FLTK implementation for GUI.

There are also two variants of executables: 'legacy' stage and stage-headless. Legacy variant works the same, with -gui option. stage-headless can only be headless, without any GUI.

Things that I have refactored to make it work:

• Canvas now wraps most rendering routines, instead of calling FLTK GL utilities. FLTK-specific routines are implemented in CanvasFLTK class.
• Menu and option system. I've stripped GUI code from Option class. All menu-related things are localized in MenuManager class. FLTK part implements MenuManagerFLTK, that links to CanvasFLTK to populate a menu
• All rendering methods get a pointer to active canvas. No need to keep a pointer to world class, just to obtain canvas pointer in rendering pipeline. It would help to implement multi canvas rendering as well.
• Image loading is localized in Image class. It uses libpng to load images. I haven't seen non-png bitmaps in world folder. Though I can consider making a more complex management of image loaders, allowing FLTK-based stage version to register additional fltk image loader.