rm obsolete `MessageBodyWidget`

cf issue #701
add MessageTree.replace_bodytext

that will make the message widget display the given text instead
of the message's body text
Merge branch '0.3.5-cleanup-message-content-701'
added check to bindings commands; added settings.get_keybindings(self,mode)
Merge branch 'bindings' of github.com:cinghiopinghio/alot into bindings