wip: resizable mailboxes list

Initial work.

see #35
Merge branch 'master' into feature/resizable_mailbox_list
Merge branch 'master' into feature/resizable_mailbox_list
Merge branch 'master' into feature/resizable_mailbox_list
Merge branch 'master' into feature/resizable_mailbox_list
Changes to fix the disappearing of the grippy icon
Merge pull request #503 from kunago/feature/resizable_mailbox_list

Changes to fix the disappearing of the grippy icon