convert App to subclass of dict to support parsing/dumping libs

Python is heavily based on its core data types, and dict is one of the more
important ones.  Even classes are basically a wrapper around a dict. This
converts metadata.App to be a subclass of dict so it can behave like a dict
when being dumped and loaded.  This makes its drastically easier to use
different data formats for build metadata and for sending data to the
client.  This approach will ultimately mean we no longer have to maintain
custom parsing and dumping code.

This also means then that the YAML/JSON field names will not have spaces in
them, and they will match exactly what it used as the dict keys once the
data is parsed, as well as matching exactly the instance attribute names:

* CurrentVersion: 1.2.6
* app['CurrentVersion'] == '1.2.6'
* app.CurrentVersion == '1.2.6'

Inspired by:
https://goodcode.io/articles/python-dict-object/

tests/metadata/org.videolan.vlc.yml

@@ -1,12 +1,12 @@
 Categories:
   - Multimedia
 License: GPLv3
-Web Site: http://www.videolan.org/vlc/download-android.html
-Source Code: http://git.videolan.org/?p=vlc-ports/android.git;a=summary
-Issue Tracker: "http://www.videolan.org/support/index.html#bugs"
+WebSite: http://www.videolan.org/vlc/download-android.html
+SourceCode: http://git.videolan.org/?p=vlc-ports/android.git;a=summary
+IssueTracker: "http://www.videolan.org/support/index.html#bugs"
 Donate: "http://www.videolan.org/contribute.html#money"
 
-Auto Name: VLC
+AutoName: VLC
 Summary: Media player
 Description: |
   Video and audio player that supports a wide range of formats,
@@ -14,7 +14,7 @@ Description: |
 
   [http://git.videolan.org/?p=vlc-ports/android.git;a=blob_plain;f=NEWS NEWS]
 
-Repo Type: git
+RepoType: git
 Repo: git://git.videolan.org/vlc-ports/android.git
 
 builds:
@@ -875,7 +875,7 @@ builds:
     buildjni: no
     ndk: r10d
 
-Maintainer Notes: |
+MaintainerNotes: |
   Instructions and dependencies here: http://wiki.videolan.org/AndroidCompile
   see http://buildbot.videolan.org/builders/ for version code scheme
   The VLC srclib commit can be found out from TESTED_HASH value in compile.sh
@@ -902,10 +902,10 @@ Maintainer Notes: |
 # +2: x86
 # +3: arm
 # +4: armv7 (CV)
-Archive Policy: 9 versions
-Auto Update Mode: None
-Update Check Mode: Tags
+ArchivePolicy: 9 versions
+AutoUpdateMode: None
+UpdateCheckMode: Tags
 # Only use higher vercode ops, if we do build those arches
-Vercode Operation: "%c + 5"
-Current Version: 1.2.6
-Current Version Code: 1030005
+VercodeOperation: "%c + 5"
+CurrentVersion: 1.2.6
+CurrentVersionCode: 1030005