Issue #3197915 by quietone, joachim, Tanuj., karishmaamin, dww:  Add details to MigrateSkipRowException messages caused by stub creation

Issue #3247040 by _shY, quietone, smustgrave, joachim: MigrateSkipRowException should document that it is for use within migration process plugins

Issue #2818799 by claudiu.cristea: MigrateSkipRowException  being TRUE, does not skip silently as should

Issue #2398467 by lduerig, tresti88, Eski, LewisNyman, emma.maria: Clean up "dropbutton" component in Bartik

Revert "Issue #1842140 by mdrummond, tonystar, gavin.hughes, killerpoke, lauriii, droweski, stevector, omg-its-maggie, jenlampton, Renee S: Remove title and wrapper div from theme_item_list."

This reverts commit 79395676.

Issue #1842140 by mdrummond, tonystar, gavin.hughes, killerpoke, lauriii, droweski, stevector, omg-its-maggie, jenlampton, Renee S: Remove title and wrapper div from theme_item_list.