New extensions sytem, WIP.

Add a new extensions systems to MO2:

• extensions can add translations, themes or plugins to MO2,
• extensions are fully isolated, no more stuff in common folder (translations, stylesheets, plugins/data),

The main goal is to add more flexibility to the current plugin system, by including themes and translations, and to allow users to install extensions for a "Marketplace", similar to the one existing for Python. Hopefully, this will let us update extensions faster than MO2 to release fixes and new features.

There are 4 type of extensions:

• theme - Provide one or more main themes for MO2, such as VS15 Dark or Skyrim, see the vs15-dark-themes and skyrim-theme archives.
• translations - Provide one or more main translations for MO2, see base-translations which contain translations for MO2 and Qt.
• plugins - Provide one or more plugins (old-style plugins), alongside translations and themes (for existing themes).
• game - Provide support for one or more game (basically a plugins extension for now).

Each extension is a folder in the extensions folder, with a mo2-metadata.json file describing the extension. The goal is to use that file to create the marketplace.

Changes

• PluginContainer is replaced by PluginManager - The main reason was to start from scratch to avoid issue, but it's very similar to PluginContainer.
• IPluginProxy is replaced with IPluginLoader, that is NOT a IPlugin. There is a standard loader embedded in MO2 (ProxyQtLoader), and the python proxy is now at plugins/proxies.
  • Note: Having proxies as IPlugin was a PITA for not much since no one is going to write a proxy without us helping him, etc.

TODO (probably incomplete)

• Implement the 4 extension types in MO2.
• Remove hard-coded stuff for the proxies.
• Remove old stuff from the plugin interfaces (description, authors, etc.).
  • Note: name(), localizedName(), settings() should be the only remaining IPlugin methods.
• Re-implement the settings for the extensions.
• Implement the marketplace stuff.
• Update mob and CMake stuff to put files in the right place.

Examples of extension:

base-translations.zip
game-tes-fallout.zip
installer-manual.zip
skyrim-theme.zip
vs15-dark-themes.zip