Add helpers for extending properties to proptools

It is common for a mutator to append or prepend property structs
together.  Add helper functions to append or prepend properties in property
structs.  The append operation is defined as appending string and slices
of strings normally, OR-ing bool values, and recursing into embedded
structs, pointers to structs, and interfaces containing pointers to
structs.  Appending or prepending the zero value of a property will
always be a no-op.