handle case when optional callback isn’t implemented
add TestingToolkit submodule to provide CTKCallendarSupplier
change search path to be relative to PROJECT_DIR
fix animation and clipping issues

- split event handler and DetailViewProvider into 2 protocols
- introduce more detail view-related callbacks
- query DetailViewProvider for height
- make the whole calendar a layer-backed view
- order rows by z-index: detail views below the rest
- change slide animaton to fit the movement
add BorderedRowView to inheritance tree

- both WeekRowView and DayDetailRowView have the same separator drawing settings
- MonthRowView won’t draw a separator
add BorderedCellView which is also used for the last blank cell in a line

- require -allowsEmptyCellSelection in detail view provider
- make DayCellView inherit from BorderedCellView
- add background color property to DayCellView, setting the value in Interface Builder
fix test target settings
guard selection outside bounds