Added local packages support.

Now Package Manager window will have additional switcher between remote and local packages. Local packages will become available whenever path to folder with local packages source codes is specified. This path is saved in Alcatraz settings file, path to local packages is relative.

Details.
1) Added Alcatraz settings file, saved in `kATZPluginsDataDirectory/kATZSettingsFile`.
2) Updated UI:
- added switcher [Remote | Local] that filters table with available packages,
- to Local tab added path selector at the bottom to specify folder with available local packages.
3) Added new fields support in project plist file:
- local package description,
- local package category.
4) Added new property to ATZPackage: `localPath`.
4) Disabled automatic downloading/updating of local packages.
Open folder with package rather then website for local packages.
Open screenshot with name `ATZPluginScreenshot.png` in project root directory if exists for local packages.