added dummy implementation of OnCarryHeavyChange callback to inventory bar
custom Gui: added "Player" property, for standard visibility

This was solved via Target+Visibility before, but is such a main feature (more guis will be for a specific player than for everyone) that it legitimates an own property
custom GUI: removed "CreateCustomMenu" which was only a wrapper for "CreateObject" - use "CreateObject" now

This should reduce the confusion about why the function behaves so differently from Gui_Open: the style objects only provide an interface for those easy GUIs, they are not really part of the GUI system.