jquery.textSelection: re-add iframe check due to regression

This is to fix a regression due to change Ibe011b2b of bug 29328.
The replacement jquery.textSelection of CodeEditor was no longer able
to be disable don the fly. This is because the code of textSelection
and WikiEditor is still too much intertwined.

I'm making a new patchset to strip them further apart.

Bug: 68556
Change-Id: I7c4c43ba2cde6f7c869e41ed4c5a51783cd32f82
Merge "jquery.textSelection: re-add iframe check due to regression"