Cleanup: wgt2allg unit

Removing obsolete variables and functions, moving some to other code units.
Cleanup: removing wgt2allg.h include where possible
Cleanup: util/file unit
Common, Engine: reworked Bitmap class

Removed abstract Bitmap interface and renamed AllegroBitmap to Bitmap.
Common, Engine: introducing Graphics struct

Graphics type gathers bitmap pointer and drawing settings, such as drawing and text colors, in one toolset object.
This is a transitional change, before incorporating Graphics class properly.
AGS.Native: using Graphics struct
Common, Engine: Graphics class

Graphics class is a step towards being more conformant with new Allegro 5 concept, according to which all the drawing is made to the preset target, as opposed to Allegro 4, where the target is defined as an argument for every drawing operation.
As of this commit, there are many cases of excessive graphics object manipulation, which reveal lack of optimization in original code. Those should be refactored in the following updates.
Graphics class uses preset color for vector drawing
Helper functions for preparing transparent bitmaps
Helper function for making a plain bitmap copy
Common: simplified Bitmap class
Merge branch 'develop_gfxremake_chpick'