[GTK] Add API to set the web extensions directory to WebKit2 GTK+
https://bugs.webkit.org/show_bug.cgi?id=106462

Reviewed by Xan Lopez.

.:

* Source/autotools/symbols.filter: Update
WebGtkExtensionManager::initialize symbol.

Source/WebKit2:

* UIProcess/API/gtk/WebKitWebContext.cpp:
(webkit_web_context_set_web_extensions_directory): Set
the given directory as injected bundle initialization user data.
* UIProcess/API/gtk/WebKitWebContext.h:
* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Add new symbol.
* UIProcess/API/gtk/tests/TestWebExtensions.cpp:
(beforeAll): Update the test to use
webkit_web_context_set_web_extensions_directory()
instead of setting the WEBKIT_WEB_EXTENSIONS_PATH env var.
* WebProcess/gtk/WebGtkExtensionManager.cpp:
(WebKit::WebGtkExtensionManager::scanModules): Append the given
additional web extensions directory to the list of directories to
scan for modules.
(WebKit::WebGtkExtensionManager::initialize): Get the additional
web extensions directory from initialization user data and pass it
to scanModules().
* WebProcess/gtk/WebGtkExtensionManager.h:
(WebGtkExtensionManager): Add userData parameter.
* WebProcess/gtk/WebGtkInjectedBundleMain.cpp:
(WKBundleInitialize): Pass initialization user data to
WebGtkExtensionManager.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139415 268f45cc-cd09-0410-ab3c-d52691b4dbfc