Register F3 shortcut to Find Step and change from popupsMenu to menus

Hi,

I moved the context menu from the org.eclipse.ui.popupMenus extension point  to use the org.eclipse.ui.menus extension point, which can use commands directly.

I have added a "Editing Feature" context to bind the keyboard shortcuts to the Cucumber editor only. And I register F3 to find step command.


This class should be rename as "PopupMenuFindStepHandler". Just wanted to keep the history for the pull request