UV panel

Add Clusters: Use LMB to mark the cluster (UV Islands) center on faces. Click the cluster again to delete it.
Mark Seams: Mark seams on the UV-Map. CTRL+LMB unmarks a seam.
Edge Loops: Mark entire edge loops on the UV-Map. Use CTRL+LMB to unmark them.
UV Path: Put path along the shortest path (under certain criteria) between several points. Click to add points or split lines, use RMB, or double-click to remove points. Press ENTER to mark seams, and CTRL ENTER to delete seams. Use different criteria to adjust path flow.
Join Clusters: Hover the seam between the clusters and click to remove the whole seam and join clusters. If the highlighted seams are red, the resulting summary islands have self-intersections or too big square trash. If it is green, the summary island has no self-intersections, and square trash is no more than 1/2.