Make GTK debug window usable again

[Tomasz Wasilczyk]

Hi,

debug window became incredibly slow since migration to webkit. It's
even not usable for some activities. I've digged around a bit, and
I've managed to speedup it really well: delay is unnoticeable.

Patch is divided to two parts:
- speeding up gtkwebview [1]: now, it gets only one element at once.
When it deals with thousands of debug log lines (typical startup for
few) it takes forever until g_idle_add hits them all. After applying
the patch, it get elements until 100ms timeout is hit.
- speeding up debug log itself [2]: adding new lines by appending html
is so slow, that timeout mentioned before is hit a lot of times. After
moving this functionality to javascript, time effort became small.

The first one seems (for me) to be ready to push into 3.0.0. The
second one needs some more work (fixing of filter feature), but it
shows the idea.

Waiting for comments,
Tomek

[1] http://pastebin.com/rWX8P6CB
[2] http://pastebin.com/iKCuf34f