conversation: update GUI every 10 mails

Each call to throttle_loading() takes ≈150..250 ms, which slows down
loading mails a lot. For now let's work around it by calling the
function less times in the cycle.

time foreach took, before 51110.635 ms
time foreach took, after   6577.519 ms

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
7 jobs for improve-conversation-loading-performance in 38 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #266234
fedora

00:03:54

passed #266235
ubuntu

00:22:28

failed #265953
fedora

00:02:31

failed #265954
ubuntu

00:00:49

 
  Test
passed #265955
flatpak

00:14:42

 
  Deploy
passed #265956
review

00:00:39

failed #265957
allowed to fail manual
stop_review

00:00:27

 
Name Stage Failure
failed
stop_review Deploy
 * [new tag]         geary-0.8.0             -> geary-0.8.0
* [new tag] geary-0.8.1 -> geary-0.8.1
* [new tag] geary-0.8.2 -> geary-0.8.2
* [new tag] geary-0.8.3 -> geary-0.8.3
* [new tag] geary-0.9.0 -> geary-0.9.0
* [new tag] geary-0.9.1 -> geary-0.9.1
Checking out ca399921 as improve-conversation-loading-performance...
fatal: reference is not a tree: ca39992139de15785a8f0ecccd1a5fa7421cb2f7
ERROR: Job failed: exit code 1