key behavior: repeat on hold, bug fixes

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implementation for #16. key hold will now work for all key presses by default. also:

• key processing extracted into its own function process_keypress
• removed global mod_ variables (for now replaced by a global variable mod_key, eventually should be probably extracted into input_state or something like that)
• fixed several bugs where pattern length wasn't properly updated in TRACK mode
• fixed a bug with [ and ] not updating values in TRACK mode
• ENTER will now always move to the next step, not just when on the last step

@tehn @samdoshi