Added documentation to the pushbutton class.
Added documentation to the page manager.
A container (if managed by a page manager) now knows the page manager
exactly.

The class has now a getPageManager() function.
Added a centered strip widget layouter.

This layout centers all containing widgets.
Added a PageManager forward declaration to fix a compiler error.
Made the frame from the button go into a member of the button.
Made the frame a member of the lineedit class.
Fixed an error in the CenteredStrip layout & added it to cmake.
Made the setBoundingBox() method of the container abstract.
Defined the static members of legui::Config in the source file.
Added 2 missing function implementations to the container.
Parents are now only assigned when the pointer is not 0.
The page manager now handles the pagecount better.

No more instant segmentation faults.
Fixed 2 segmentation faults.
Added a removeFromParent() method to the widget & fixed a destructor
segfault.
Made the container parent of the page manager public.
The button now sets its style at initialization.
Better debug facilities in the config.
Fixing some bounding box errors.
Fixed the focused outline (thickness).