Cache package list

[guillaumealgis]

Fixes #143. Crosses off an item in #201.

This saves the package list in ~/Library/Caches/com.mneorr.Alcatraz/packages.plist after the first successful package.json download (and updates it after each successful reload).
Once the package list is on disk, it will be loaded on subsequents Alcatraz launches, before an attempt is made to get the fresh json from Github. This results is instantly displayed packages at launch, even without network.

The package list is stored as binary plist for efficiency and ease of use, but JSON could be considered to keep the same format as Github. Let me know.

[guillaumealgis]

Rebased this PR on top of master. Could you take a look? Thanks!

[jurre]

I definitely like the idea of caching the package list, @supermarin what do you think?

[supermarin]

Thanks for rebasing @guillaume-algis, this seems great!

Just to check before merging - what happens when you start scrolling or searching on a bad connection and the tableview reloads?
e.g. does it preserve the scroll state and filter state?

[guillaumealgis]

Good catch. The table view lost its content offset after a reload, as the delegate was always recreated.
This is fixed in 14bf5be.

The search & filters state are preserved.

I'm not quite sure what kind of UI we should display for the install buttons, though. For the moment, they appear as if you were online, but attempting to install anything will fail 😕

[jurre]

We could disable the buttons and show a no internet connection warning somewhere I guess?