MIDI import of drums: longer notes, less rests

The goal of the new option is to minimize number of rests on drum staves for clearer notation. 
We talked about it on IRC earlier (September 28).
Also Sibelius 7 has this feature. 
The rests deletion is turned on by default.

Before - drum sets with rests:

https://docs.google.com/file/d/0B5alKuFoSol2cUkwN3p5VzVYdkk/edit?usp=sharing
https://docs.google.com/file/d/0B5alKuFoSol2Z3pCcWtmWVJ4UHM/edit?usp=sharing
https://docs.google.com/file/d/0B5alKuFoSol2TXludE5CRWpidG8/edit?usp=sharing

After - very few rests remaining:

https://docs.google.com/file/d/0B5alKuFoSol2MGJlSV96bTRfNTQ/edit?usp=sharing
https://docs.google.com/file/d/0B5alKuFoSol2Rm43RkxoY0YxaU0/edit?usp=sharing
https://docs.google.com/file/d/0B5alKuFoSol2VDNPNk5qZ3NtYm8/edit?usp=sharing