shell-app: Improve prefix matches

Currently we use a very strict definition of "prefix", where the
search term has to match at the very beginning of the searched
criteria (application name, executable name). Use a more liberal
definition by including matches where the preceding character is
a space (application name) or hyphen (executable name) as well;
as many applications use a prefix, this should improve the quality
of results.

https://bugzilla.gnome.org/show_bug.cgi?id=623372