Gesture-event interface for touchscreens

This pull-request contains some of the work I did during GSoC (It's best the different features be split into separate pullreqs.).

Generally, in ScummVM widgets and dialogs have the function handleMouseDown which is called upon the active dialog when we detect a mouse-down event (EVENT_LBUTTONDOWN), for example. These commits add events for touch events that are to be interpreted by widgets. It also features widgets that are "ported" to work with gestures (i.e. scrolling lists by swiping up/down) and enabling android to use these (configure with -enable-touchmapper). 

In order to get this to work and also to work with games a "touch-mapper" is used, which is an eventManager that interprets touch-events and translates these to either work with touchpad-mode in-game, direct-input in-game or to be interpreted normally in the GUI (also another plus, is that this does not need to be written from scratch for every port).

Anyway, this code seems to work fine for Android-devices, I'd love to get some feedback as well if other gestures should be added, changed, if it may pose problems with other touch-device ports etc.