simplify switching between flex and fixed width

since ZK 8 simultaneos usage of h/vflex and fixed height/width has been restricted -> OK

however the current implementation makes it diffcult to reset height/width without additional checks or workarounds.

The attachment shows a simple scenario with 2 buttons to switch between a flex and a fixed width. When clicking a button twice it will lead to an exception even if the intermedate results never violate the flex vs fixed constraint

The idea is to reset hflex to NULL before setting a width, and vice versa.
However when hflex is already NULL and a width exists a call to setHflex(null) causes an exception, even if it does not lead to an illegal state. Instead the developer has to perform tedious IF-checks everytime when trying to set a component to either flex or fixed width.

to set a fixed width:

//reset width before attempting to reset hflex
if(comp.getWidth() != null) { //need to check first in case hflex exists
  comp.setWidth(null);
}
comp.setHflex(null); //this would cause an unnecessary exception if width still existed
comp.setWidth("300px")

when allowing to always set NULL values those checks become unnecessary

  comp.setHflex(null); //if NULL always allowed -> no prior check necessary
  comp.setWidth("300px")

and for the opposite way the same principle

  comp.setWidth(null);
  comp.setHflex("1");

A proposed solution to allow NULL values is to apply this pattern in setH/Vflex and setHeight/Width

public void setVflex(String flex) {
   if (getHeight() != null && !(flex == null || "min".equals(flex)) { //allow null here
       throw new UiException("Not allowed to set vflex and height at the same time except vflex=\"min\"");
       }
    setVflex0(flex);
}