Move validation into assembly resolve

Move validation into assembly resolution so that it can be bypassed if
someone else resolves the wrapper
Slim down validation code
Refactored DataStorage;
Fixed data file saving after project properties update.
Fixed cross-thread access to form fields.
Added mechanism for passing callbacks from C# to C++ with delayed call;
Receiving notifications about all files opened by libsass.
Merge pull request #2 from darrenkopp/assembly_resolve

Move validation into assembly resolve.
Fixed tabulation for last pull request.
Added UnsetCallback to CallbackManager;
Added file checksum calculations;
Changed log text area to read-only mode;
Added preprocessing logic including dependency tracking (without physical files saving for now).
Merge branch 'master' of https://github.com/TBAPI-0KA/NSass