add Is Element keyword

[tkopra]

Is your feature request related to a problem? Please describe.
Quite often there is a need to add conditional logic to a test based on UI state. Currently I need to do some ugly Run Keyword And Return Status or TRY/EXCEPT stuff or use Get Element Count as workaround.

Describe the solution you'd like
new keyword Is Element that returns boolean value whether element defined by selector is found or not.