[trunk] Extra rests in measure when entering (some) notes

Setup: rev. 4304; Ubuntu 11.04, Qt SDK 1.1 (Qt lib 4.7.3) / Windows XP Qt SDK 10.04

Steps:

1) Load the attached sample score
2) add a new semibreve right after the first note in the first stave

Result: After adding the new semibreve, the rest remaining in the measure is not a single minim, as it should, but 1 crochet + 1 minim.

Notes

1) Two (or more) staves are necessary: with only one stave, the issue does not show up.

2) This happens since rev. 4304 and was NOT happening in rev. 4303.

Priority set to critical, as it leads to data corruption.