Adding page up, page down, and filter to the file browser

[hcgernhardtiii]

File browser functionality changes:

• Added page up/page down facility (shift-up and shift-down arrows)
• Added filter facility
• Fixed filter display bug on file browser header rows
• Updated menu bar to reflect paging option

Quality-of-life changes:

• Makefile.coco now has a variable to denote where to put autorun.dsk

[tschak909]

Thank you, good sir.