Speed up grep output view.

Grep output view is very laggy if there is a lot of items. For example
on my machine adding just 300 items to the view takes about 10 seconds
and whole KDevelop UI becomes unresponsive. After applying proposed
changes there is no problem to add even 10 000 items - everything
works without any lag.

Also, I didn't notice any regression in the UI: all rows in the view
are displayed properly.

REVIEW: 124003