This patch introduces the basic key handling required to browse through
a multi-page PDF document ([page-up] or [enter] -> next page,
[page-down] or [backspace] -> previous page).
This way, we can register a meaningful 'user_data' pointer to at the
'pdfapp_t' structure and avoid the use of singletons for 'Framebuffer'
and 'pdfapp'.
This version of MuPDF will use Genode's framebuffer session and input
session to display a PDF file. At the current stage, the program is
merely a skeleton that compiles and starts. Neither the actual rendering
not the response to user input are implemented. To try it out, there
is a ready-to-use run script at 'libports/run/mupdf.run'.
