introduce waitForImageCase

[simonmeggle]

A dialogue which is apperaring "sometimes" can only be catched in this way:

if (screen.exists("dialog",30))  {
  screen.click("okbutton");
}

If the dialogue is not apperaring, Sakuli has to wait 30 seconds - useless.

waitForImageCase(
   "bild1", sec, _functionhandler1,
   "bild2", sec, _functionhandler2
);

would solve the problem. Sakuli would wait for two images at the same time and execute one of two function handlers.

[toschneck]

implement after #26 and callbacks are possible