RCORE: fix missing "virtual"
UI: properly handle critical condition as such, fix debugging
UI: serverinsert: remove unused structure helpers
UI: buttons: fixed missing 'virtual'
UI: cosmetics
UI: widget: add unset_focus
RCORE: associate debugging backtrace snapshots with keys for deferred retrieval
UI: container: chain up from dump_test_data
UI: widget: provide root() to find the toplevel of a widget's ancestry
UI: button: chain up in dump_private_data
UI: widget: introduce debug_dump() to debug states of widget trees
UI: widget: add requisition, allocation, clip_area to test dumps
Merge branch 'debugging'

* debugging:
  UI: widget: add requisition, allocation, clip_area to test dumps
  UI: widget: introduce debug_dump() to debug states of widget trees
  UI: button: chain up in dump_private_data
  UI: widget: provide root() to find the toplevel of a widget's ancestry
  UI: container: chain up from dump_test_data
  RCORE: associate debugging backtrace snapshots with keys for deferred retrieval
  UI: widget: add unset_focus
  UI: cosmetics
  UI: buttons: fixed missing 'virtual'
  UI: serverinsert: remove unused structure helpers
  UI: properly handle critical condition as such, fix debugging
  RCORE: fix missing "virtual"