[fix] use lazy redraw for smooth transition
[minor] remove global mappings, use local buffer mappings for each one
[minor] set winfixheight
[minor] change default highlight group
prevent redundant plugin load

- prevents errors from eg BundleInstall
Add code to support having GUI menu options and bitmaps for buttons
[minor] make sure resize and winfixheight
buffer-local mappings

also add '<leader>do' synonym for consistency with Vim diff built-in
'do'