Script: enhanced function callstack code a bit
Script: started implementing Script API

+ AudioChannel API
+ AudioClip API
+ Button API
+ Character API (few functions left)

WARNING: project is not-compilable at this point
Script: implementing Script API (in progress)

+ DateTime API
+ Dialog API
+ DialogOptionsRendering API
+ DrawingSurface API
+ DynamicSprite API
+ File API
+ Game API (partially)

WARNING: project is not-compilable at this point
Script: implementing Script API (in progress)

+ Math API
+ Mouse API
+ Object API
+ Overlay API
+ Parser API
+ Region API
+ Room API
+ Slider API
+ String API
+ System API
+ TextBox API
+ ViewFrame API

WARNING: project is not-compilable at this point
Script: implementing Script API (in progress)

* Distinct types for static and object API functions
+ Global API

WARNING: project is not-compilable at this point
Script: implementing Script API (finishing)

+ Built-in Plugins API
+ GUI API
+ GUI Control API
+ Hotspot API
+ Inventory Item API
+ Inventory Window API
+ Label API
+ List Box API
* Fixes & cleanup

WARNING: project is not-compilable at this point
Script: distinct ways to call script functions

Still to be done:
1. Properly implement sprintf-style script functions
2. Remove GlobalReturnValue from engine functions
Script: implemented ScriptSprintf() function
Script: fixed trailing semicolons in macros
Script: simplified function callstack
Script: made API function parameters const
Script: GlobalReturnValue is used only by plugins
Removed function my_sprintf(), as it is now obsolete
Script: fixed CALLAS
Script: fixed Sc_DisplaySpeechBackground()
Script API: fixed few more incorrect return values
Script: simplify pointer arguments only when calling engine functions

The previous change to function callstack was a mistake: pointers should not be converted to simple types when script function is called, they must keep their original type so that script handle them properly.
Merge remote-tracking branch 'ags-hub/master' into script_api
Fixed compilation on Linux
Script: safety checks for Instance stack