Added the ability to "inherit" constraints when extending a style

The logic ensures that only one constraint for an attribute will be applied in the new style

It is not perfect:  if you use a symbol in the base style such as `:top_left` and `constraints_top(8)` in the extended style
both will exist incompatibly.
Merge branch 'extending-constraints' of github.com:alexrothenberg/teacup into alexrothenberg-extending-constraints
added specs for Symbol-based constraints