Replace exo_str_* () with GLib functions

Related : exo#82

Requires GLib >= 2.66.0

exo_str_is_equal => g_strcmp0 () == 0
exo_str_looks_like_an_uri => g_uri_is_valid (uri, G_URI_FLAGS_NONE, NULL)
2 jobs for !236 with g_string_replace in 7 minutes and 22 seconds (queued for 2 seconds)
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