Implement the function table in GUI2 formulas.
Add reevaluate_best_size_ function for a window.

This function will be used in the tooltip window to evaluate whether the
final window size is proper. It also lets the window really handle
functions.
Add a new GUI2 tooltip.

The placement of tooltip is controlled by the WML of the GUI engine. The
code is not used yet, only available with a proof-of-concept define named
DEBUG_TOOLTIP.