Add function to BaseComponentBuilder for defining a content-based size

Current behavior

Currently the API provides a 'size' function which defines the size of the component, e.g. Panel, inclusive artifacts like borders and shadows.

Improved behavior

Since the API user often is more interested in defining the size of the actual content of a component, it would be nice to have an add

This is related to #226, but at a lower level, improving the building blocks for widgets.

What is the current behavior?

When you are on top of an NPC and want to trade/shop/bank with them. The popup screen will not popup UNTIL you move and FINISH its progress.

**If the current behavior is a bug, please provide

Would be very effective against regenerating foes (then we can add and/or improve foes that are very weak, but regenerate and possibly avoid melee to be safe from big hits that they can't regenerate against). Can be a throwing weapon, because swapping it in and out as a melee weapon worth it only for regenerating foes could be too tiring.

Low-priority. cppcheck complains about null pointer arithmetic in the filelist functions in platformdependent.c. It would be good to re-write them to avoid it

src/platform/platformdependent.c:471:54: error: Pointer addition with NULL pointer. [nullPointerArithmetic]
    list->files[list->nfiles].path = ((char *) NULL) + list->nextname; // don't look at them until they are transferred out

md docs are incomplete

examplegame startup doc is out of date