[applets/notifications] Try to not have the notification count numbers overflow the icon

Summary:
When the notification count is in double digits, the numbers can overflow the icons which is meant to contain them.

This patch checks if there is more than 1 digit and then slightly reduces the size of the numbers.

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22878