Use getIdentities

After the change in stdlib I changed all gIdentities-calls to getIdentities/getDefaultIdentity.
I have to admit that I didn't test all changed methods thoroughly as I don't know when they're called.
I also tried to call toLowerCase() on all strings that were compared with an email-address in an identity.
This function is called when you click on the small arrows in the quick reply next to the email address you're using for sending the message. The effect is that we cycle through the list of available identities for sending that email. `dir` is either 1 or -1.

It can be simplified by keeping a reference `i` and just doing:

`this.params.identity = getIdentityForEmail()[(this.i = this.i+dir) % getIdentities().length]`

(or something like this)