Fixes #869

Allow creating a mix when we don't have the full default mix duration available at clip ends (minimum of 3 frames now required)

Fixes #851

Fix undo import subtitle file, and improve subtitle group operations (only reload subtitle file once)

Fix subtitle model used start time as index - caused issues if a subtitle if moved after another one. Add start/end position control in subtitle widget

Relate to #796

When switching project profile and there is only 1 clip in timeline, update the timeline clip duration accordingly to profile change.
Fixes #819

CCBUG: 403443

CCBUG: 403443

Small cleanup, make placeholder timeline clips visible and allow reloading missing clip if source file is available
Fixes #785

Allow easy add of a mix by double clicking a clip's resize handle

Related to #796