Bugfix ofToDataPath using Poco::Path

Replaces #1251

Closes #1522, #1250, #819

Needs serious testing across all platforms.
Works fine here on osx first time (so i might have overlooked something)

Using Poco::Path we seriously clear up a bunch of folder functions.

I've also introduced ofSetWorkingDirectoryToDefault() and moved the call for it to ofRunApp, which should resolve #1522

Also I'm starting to think that ofDirectory should be returning Poco::Path derivatives not Poco::File derivatives.

I'm not sure here if windows system dialogs changing the cwd are fixed. Will look more now, but eyes on early!