Fix UiSelector instance in find_elements

If a UiSelector already uses instance, then invoking instance again
will replace the original instance number. The UiSelector will then
point to a completely different element.

To fix this problem, these selectors ending with instance are detected
via a regex and the extra .instance call is avoided.
Merge pull request #3730 from bootstraponline/fix_uiselector_instance

Fix UiSelector instance in find_elements