create app in separate source directory outside Gaia profile

[mykmelez]

r2d2b2g's nascent "Create App" function creates an app in a webapps/{UUID}/ subdirectory of its Gaia profile directory, which it uses to store both source files and install files. This is problematic for several reasons:

1. the files will be deleted when the user updates the addon, since the addon currently completely deletes and replaces its Gaia profile on update;
2. the files are located in an obscure location, so users will have difficult finding them outside of an Open File Location function within the addon;
3. there's the potential for source and install files to conflict;
4. developers can't choose the location of the source files.

Instead of creating the app in a subdirectory of the profile, the Create App function should create it in a subdirectory of a common location (like the user's default Documents directory) or let the user pick a location. One might combine these two with code that uses the file picker to prompt the user for a name while suggesting both a name ("My App") and a location (using Services.dirsvc from Services.jsm to get the location via the platform-specific definitions in nsDirectoryServiceDefs.h).

Once the source files are in a reasonable, potentially user-specified directory, the rest of the code, refactored, can behave much the same way: installation consists of packaging the files into a ZIP archive, installing the package into a webapps/{UUID}/ subdirectory of the Gaia profile directory, and adding an app record to webapps/webapps.json.

[digitarald]

With #44 this should be fixed, as all locally stored apps are now re-installed on update. I'd close it for now, as its not in the spec.