[desktop]Fix keyboard issues for desktop B2G client

Regarding issue #489 , we have added wait_for_element_displayed in the Keyboard class so it won't failed by trying to tap something too fast.

This is dependent on [bug 852456](https://bugzilla.mozilla.org/show_bug.cgi?id=852456) and subsequent Marionette client release, as it had issues with double_tap and long_press.