webelem.text() was only used without use_js=True from webelem.__str__.
Now we instead default to the use_js=True behavior and renamed the
method from text() to value().
The old behavior is instead directly implemented in __str__.
This changes how webelem.value (aka text) handles elements for which
is_content_editable() returns True, but I haven't found any cases where
this makes a difference.
This also fixes getting existing text from elements with QtWebEngine,
which closes#1957.
