Switching buffers in the standard ctrl-tab way

[onelivesleft]

Allows the user to switch between recent buffers, which should be an orthogonal operation to the cursor history (current move_to_previous_buffer/move_to_next_buffer use the cursor history). This organises the buffers in order of recent access, per pane (i.e left / right editor). While choosing a buffer, the filenames are drawn as tabs in the status bar.

Normally editors would bind this to ctrl-tab/ctrl-shift-tab; I've left the defaults for it as ctrl-pagedown/pageup for consistency with the current keymap, but would recommend rearranging these (and moving the current switch_between_open_files dialog to something else). In this PR the ctrl key is hard-coded: once the user starts flipping through open buffers, we stay in this mode until the user releases ctrl.

Also added close_current_buffer (default Alt-w)

Other changes in this PR:

• Switched most_recent_buffers to our Array type.
• Renamed Tween_Animation.speed to Tween_Animation.duration
• draw_file_info now takes a flags parameter which governs which things to draw

[onelivesleft]

This is usable as is, but needs a bit of polish on close_current_buffer when the buffer is modified, which I'm working on now

[onelivesleft]

close_current_buffer polishing done.