Incremental clean up of paths without trailing "/":
- Rename FileUtils.canonicalizeFolderPath() to stripTrailingSlash() since
it actually makes paths NOT canonical. Old API left in place with
deprecation warning for now.
- Add warning to docs for a few other APIs that return non-canonical paths:
FileUtils.getNativeBracketsDirectoryPath(), FileUtils.getNativeModuleDirectoryPath(),
ProjectManager.getInitialProjectPath(), ProjectManager.updateWelcomeProjectPath()
ProjectManager._getWelcomeProjectPath()
- Indicate explicitly which ProjectManager APIs support receiving
non-canonical paths
- Centralize code that touches welcomeProjects storage pref for clarity

What this does NOT do:
- Fix the other APIs listed above that still return non-canonical paths
- Fix non-canonical first-launch project path given to _loadProject()
- Fix non-canonical paths used in RecentProjects prefs
- Fix non-canonical paths used in welcomeProjects pref
- Fix ScopeManager, which in some places requires canonical paths and
in other places requires non-canonical paths
Fix some APIs to work with canonical paths (ending in "/") instead of non-
canonical:
- ProjectManager.getInitialProjectPath() - always returns canonical
- ProjectManager._getWelcomeProjectPath() - always returns canonical
- ProjectManager.updateWelcomeProjectPath() - arg assumed to be canonical &
always returns canonical

None of these APIs are used in extensions.

This does NOT change the underlying "welcomeProjects" pref - like
"recentProjects", it still stores paths without trailing "/" under the hood
Code review: Consolidate code that adds trailing slashes; make empty-string
behavior more explicit
Merge pull request #5527 from adobe/pflynn/trailing-slash-cleanup

Better compartmentalize code that assumes no trailing "/"