#350408 by pillarsdotnet: Selecting "Regenerate marker cache" does not reliably clear and rebuild the marker cache.
Changes:
gmap_regenerate_markers()

    * Pass TRUE to the gmap_get_icondata() function
    * Pass TRUE to the gmap_set_marker_titles() function

gmap_set_marker_titles()

    * Simplify and correct the caching logic

gmap_flush_caches()

    * Add an implementation of hook_flush_caches to ensure that clicking on the "Clear cached data" button from the /admin/settings/performance page also rebuilds the marker cache.