Bug 928968 - Add a command to the gaia command line tool to print the applications installed

Adds an `installed_apps` property to `GaiaApps` and a `listallapps` command to gcli. Also modified the `runningApps` method to be a property and return a list of `GaiaApp` objects. I have updated all places this method is called, and checked that it's not used in any package (that I'm aware of) that depends on gaiatest.

Output looks like:
```
$ gcli listallapps
1: Browser
2: Calendar
3: Camera
...
```
```
$ gcli listrunningapps
1: Homescreen
2: Messages
3: Phone
...
```