Change signal for hasAccounts to emit when accounts are added or removed

This fixes the bug where the global drawer won't show up (caused by !139 (merged)) when logging in because it's bound to AccountManager.hasAccounts. However, that property NOTIFY is for accountsRemoved, which of course doesn't happen when logging in. Now I created a new signal dedicated for that property, and emitted in the same places as accountsAdded and accountsRemoved and the global drawer should now properly show/hide itself.