iconGrid/iconGridLayout: Make sure to always update best size

When selecting the best icon size for the available area, we
iterate from the biggest icon size to the smallest one, and
stop when finding a size that fits the available area.

However, the 'bestSize' variable is only updated when the
available area is positive. This is problematic in super bad
cases like when none of the icon sizes actually fit the availabe
area, which was hit with a previous iteration of this branch.

Make sure to update the best size while iterating, so that the
smallest size is selected even in such bad cases.

Part-of: <!1510>
11 jobs for !1510 with gbsneto/page-padding in 3 minutes and 39 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Pre Review
passed #1018443
check_commit_log

00:00:21

passed #1018444
check-merge-request

00:00:37

 
  Prep
passed #1018445
build-fedora-container

00:00:42

 
  Review
passed #1018447
eslint

00:01:00

passed #1018448
eslint_mr

00:00:54

passed #1018446
js_check

00:00:24

passed #1018450
no_template_check

00:00:30

passed #1018449
potfile_check

00:00:29

 
  Build
passed #1018451
build

00:02:25

passed #1018452
flatpak
flatpak

00:01:44

 
  Test
passed #1018453
test

00:00:51