Strip and collapse spaces from `Element Text Should Be` and `Element Text Should Not Be`

[jannek76]

Sometimes element has spaces that can be accepted for the test case. Specially leading and trailing spaces can occur in element.

These functionalities has added to BuiltIn Should Be Equal keywords in RF 4.0 and RF 4.1 and would be nice to have in SeleniumLibrary too.

PR: https://github.com/robotframework/SeleniumLibrary/pull/1749