Allow scrolling in credits screen with up/down keys

[martinvanzijl]

Fixes #1231. You can now scroll in the Credits screen using the up and down arrow keys. You can also skip ahead using the action or jump keys.

Demo below. Here I use the down, up action ("a" for my setup) and jump (space) keys:

[martinvanzijl]

Further notes - there seem to be two scenarios in which a text-scroller object is used:

1. A "textscroller" element is added directly to the level file. This is done in the Credits screen.

2. Using the script function ("display_text_file("). An example of this is the first level in "Tux Strikes Back". This creates a new TextScrollerScreen object.

Keyboard handling was mostly done in TextScrollerScreen, but I have moved this to TextScroller, so that it works in both scenarios 1 and 2.

[tobbi]

Looks good!