Fix crash when moving from a 1-argument trigger to 0-argument trigger.
Remove trigger field focus.
Clean up drawing of selected triggers.
Fix following of cursor in trigger rows.
Merge pull request #169 from kagu/fix_trigger_focus

Fix trigger focus
Fix conversions between pixel offsets and timestamps.
Implement vertical zoom.
Support very large vertical scroll ranges.
Keep edit cursor visual position fixed when changing zoom level.
Fix rendering of end of sheet with very deep zoom levels.
Support basic column width adjustment.
Configure a fixed set of available column widths based on font size.
Keep edit cursor visible when changing column width.
Use a buffer cache that supports limitation of memory usage.
Limit column cache size.
Fix cursor movement to a previous system.
Remove unneeded sync.
Store sheet zoom settings in the session.
Implement sheet manager for accessing sheet settings.
Use the model for accessing sheet zoom parameters.