[packagekit] renice worker threads to idle priority

[Bill Nottingham]

On any loaded filesystem, heavy disk I/O (including fsync()) tends to
grind the system to a halt. What do the guts of most package managers
do? Heavy disk I/O.

I suggest that PK re-nice all its worker thread (metadata reading/compiling,
package installing, etc.) to idle priority, so they don't completely
bog the system down.

Bill