Element Should Be Focused keyword should use driver.switch_to.active_element also with Firefox

[aaltat]

With selenium 3.6.0 (or smaller) and when using Firefox (with geckodriver) this driver.switch_to.active_element returns a dictionary which contains the a WebElement. Starting from Selenium 3.7.0 this is fixed and driver.switch_to.active_element returns directly a WebElement. Change the SeleniumLibrary:

1. Use driver.switch_to.active_element always
2. Support both Selenium 3.6.0 and 3.7.0

[pekkaklarck]

Element Should Be Focused was a new keyword added in 3.0 (#851). Fixes to it don't really belong to 3.0 release notes.