Update UI style, add package reloading and URL customization


## Changes

* Remove bindings in favor of separate delegate/datasource in `ATZPackageTableViewDelegate`
* Moved window xib to controller, separated cell xib file
* Restyle cells to include icons for git hosts, inline previews, new colors
* Create new filter bar with option to show only all packages or installed packages, filtered by type
* Added configuration menu for reloading the package list, changing the package source, or reseting the package source to default

Updates #201