vim: add helper to get current word
whitespace
source-view: add request-documentation and display-documentation signals

request-documentation is a G_SIGNAL_ACTION that can be activated from
a GtkBindingSet. The attached keybinding is <Control><Shift>k.

We also wire up the <Shift>k accelerator from VIM to use the same
infrastructure.
source-view: whitespace
editor-frame: wire up to display-documentation signal