Small update to RiscLua wimp library

Slight changes to RiscLua’s wimp library. The examples and documentation have been updated accordingly. There is a new library screen to which the window.centred function has been moved. There is a new function template.install which creates and attaches to the task all the windows in the template file. Its use means that window.init and statements to attach windows no longer have to be made explicit. In practice this means that the window library does not need to be used explicitly.
Wimp programs come down to binding three things together: locations, triggers and actions. Locations are (window-icon) pairs; windows being given by names in the template file, icons by numbers. Triggers are clicks or drags. Apart from a few standard predefined actions (open/close window, alert, open a menu, send a message, quit, .. ) actions are user-defined handlers, taking the whole task as first argument.