Assign explicit values to serialized enum constants
Removed unused STAIconShowingMode enum
Move doc set indexing and loading to STADocSetStore

The doc sets available on the file system now control what doc sets are
loaded at launch. Indexing and symbol cache storage are placed under the
DocSetStore to facilitate moving to database symbol storage.
Use localizedStandardCompare: for sorting indexing doc sets
Save disabled doc sets rather than all enabled doc sets

This enables newly-discovered doc sets by default and simplifies
preferences management associated with enabling/disabling doc sets.
Update no documentation message and positioning
Display indexing progress

Also make use of cell view reuse identifiers so there is no unnecessary
view creation.
Load cached doc sets at launch, compare against available doc sets

Re-index if a newer version has been installed.
Differentiate between symbols not loaded and no symbols
Automatically handle added/removed/updated doc sets
Remove cached indexes for doc sets that no longer exist
Send docSetStoreDidUpdateDocSets: for the initial load from cache
Remove unused parentName from STASymbol
Support relocatable doc sets

The symbol now store a path relative to the doc set URL and the doc set
URL is a file reference URL. This allows the doc set to be moved/renamed
while the app is running or between app launches and the path to search
results will continue to be resolved without a need for reindexing.

Not dealing with NSURLs for each symbol also significantly speeds up
loading of the cached index.