* Fix update and play limits for play-selection mode when not looping * Fix playback in loop mode when no selection -- but the GUI update for this is still wrong on the flyback * Various fixes and improvements to making selections, particularly during playback * Draw selection under non-opaque non-scrollable layers, so as to improve cacheing * Show selection limits as text when drawing selection * Allow user to find missing audio files when loading session * Cross-fade selections when in play-selection mode -- mostly. We don't cross-fade on a processing block boundary, and unfortunately with short selections the selection boundary is quite likely to coincide with a block boundary.