Replaces libinterface's ToolButton, a custom button that can display
a pop-up menu on click (along with a custom image). It didn't scale well
with different themes and font-sizes compared to BButtons― which
ToolButton is supposed to look like to begin with.
A BButton-derived class is used instead, so these buttons look more
in-place among other buttons.
Accounts can now be temporarily disabled (in a Pidgin-like style)
through Preferences->Accounts. Work is still required to allow
enabling/re-enabling of accounts on-the-fly, and for keeping an
account's disabled state persistent.