Extract keys.c:execute_after_remapping()

Processes remapping of a key.
Handle empty rhs in engine/keys correctly
Display empty rhs in :map menu as <nop>
Fix braces in map_menu:add_mapping_item()
Add <nop> angle bracket notation
Better string.h include in main_loop.c
Highlight <nop> in syntax file
Document addition of <nop> bracket notation
Merge branch 'add-nop-braket-notation'

Added <nop> angle bracket notation.

Thanks to filterfalse.