Fix spilling messages in sequence diagrams.

[slavc]

Long messages in sequence diagrams do not fit between boxes.

Fix this by putting space between boxes equal to the width of the longest message between those boxes.

[slavc]

suml --scruffy --sequence --png '[Driver]set key pressed>[Keypress buffer],[Keypress buffer]wake up>[HID layer],[Driver]unset key pressed>[Keypress buffer],[HID layer]retrieve>[Keypress buffer],[HID layer]send key press data>[Host]' > test.png

Before:

After:

[aivarsk]

Respect!