Restore keyboard shortcut for expand all diff context
In Ib3fe44d502a the keyboard shortcuts system was redesigned, but the
binding for Shift+x shortcut for expand all diffs was lost. Restore
this binding and confirm that it is now listed in the help dialog and
works as expected.
Bug: Issue 13054
(cherry picked from commit 956cf9a0aa3d38905762853bca8dbf3766170bcc)