QBaseClass: Add variable management functions to sub-classes

This class can be used to add automatic variable management functions to classes.

Its default class function handler (__call) is defined in such way that it adds new automatic functions to the class to manage class variables.

The automatic functions are recognized by their name prefixes. Currently it supports:

- has for checking if a variable is defined
- set to defined the value of a variable
- get to get the value of a variable
- add to set the value of an entry of an associative array variable
- merge to change the values of multiple entries of associative array variables