AccountDialog: Remove account from startup accounts on deletion

Woopsie.
Add show-account-dialog accelerator
ui: Add some tooltips for the new account stuff
AvatarBannerWidget: Use fixed banner size ratio

Banners are always 640x320 so we can just assume that and fix some
sizing issues if no banner is loaded (i.e. in the case where an acconut
has a custom banner set).
account-dialog: Refine the text view

Use a GtkFrame to make it more visible, don't accept tabs and wrap at
word-char.