Rename `get_binding` to `get_keyboard_binding` and ensure that this
function only accesses bindings of type B_KEYBOARD. Other types of
bindings (e.g. mouse bindings) will be accessed by a different function.
Create files bindings.[ch] to contain functions for configuring,
finding, and running bindings.
Use the new function `configure_binding` for binding configuration. This
function adds a binding from config parameters.
Export the function `modifiers_from_str` from config_directives.h.
This change is made in preparation for the new bindmouse functionality.
