Sets the priority of the tag to priority. Valid priority values are 0 through one less than the number of tags in the GtkTextTagTable.
Priorities are used to resolve conflicts when two tags attempt to set the same attribute across the same range of text. The values of the tag with the higher priority will take precedence over the values of the tag with the lower priority.
Priorities are unique for all tags in a given GtkTextTagTable. By default, priorties are assigned in the order the tags are added to the table. If the priority of a tag is changed, all other tags' priorities are adjusted.
See also: get_priority()