Webkit2 implementation of getText().

Implementation is similar to evaluate(), in that
it converts async call to sync by waiting for async callback.
It piggybacks on evaluate's async-callback framework. 

As both use the same framework, I abstracted the framework from
being specific to 'evaluate' to a generic 'asyncToSync'.

Test:
- Checkout this patch
- Cherry pick jUnit patch: (if not merged yet).
  https://git.eclipse.org/r/#/c/96984/
- Run Test_org_eclipse_swt_browser_Browser
- All newly added getText() jUnits pass on Webkit2.

Gtk3.22, Webkit2.16.1

Change-Id: I0a1d4e94b3a78abba4be9acb5a3f43311d023059
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=514719
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>