This value is defined in the UI module, but happens to be used
in string_search.cc too. Note that these references need to be kept in
sync. Use escaped utf-8 sequence since the literal can be avoided.
Also replace BLI_str_utf8_as_unicode calls with constant assignments
as these values are known there is no need to decode a utf-8 sequence.
This adds a generic string search library in `BLI_string_search.h`.
The library has a simple to use C api that allows it's users to
filter and sort a set of possible search results based on some
query string.
Reviewers: Severin
Differential Revision: https://developer.blender.org/D8825
