Write sequencer module

All the step sequencer global state is now inside a struct.
Add "make zip" task
Bump version to 0.8
Add missing comments in main app loop
Move all step advancement code to seq_tick()
Write Sequencer UI module
Refactor as MVC

Define 2 controllers, one for each mode (pattern editor, instrument
editor). Sequencer is the only model (for now), and each mode has its
own view module.
Add nicer default INSTRS.DAT
Move Makefile to root and font files to fonts/
Add XCF (Gimp) source files for fonts
Pattern Editor View
Rename pe_view.* for pe_vw.*
Instrument Editor controller and view modules
Tidy main.c a bit
Define seq_toggle_step() and seq_toggle_microstep()
Define seq_set_instrument()
Remove const in ie_vw for seq model
Add make dosemu to fireup DOSemu (faster to compile)
Make vga module follow current naming convention
pbm: naming convention