Sorting directory entries now puts directories before files and treats
upper- and lower-case as equal. Displaying file sizes is implemented as
prototype and now yet working correctly. Plus there is a new bug
regarding entry selection.
Started restructuring the modules vm and nav, hopefully with the result
of easier to maintain code that is more flexible to use. This round the
fill_window function got replaced by the much cleaner function draw_dir
which is capable of drawing any subset of directory entries depending on
the position of the selected entry. It uses a very simple and much more
elegant algorithm for that.
Simplified vm.lua drastically and fixed a bug in the draw_dir function
for directories with lots of entries.