Added search-engine support.

Added a command to add and remove search-engines and extended the open command
to query the search-engine if the shortcut was found as first part of the open
or tabopen command.
Added a '_' before the include guards.

This is not required, but seems to be the common practice.
Adapted the year in the license headers.
Simplified keybind by removing string splitting.

The splitting of keybind input string is now done in the command that maps the
keybindings.
Replaces the gchar -> char and gint -> int.