Bug 948622 - Improve activate_edit_mode()

I updated the locators here and the wait. Not sure if you like _visible_icons_locator, but I think it looks better than the old locator. Another way to write this locator would be by using nth-child. 
I ran the affected tests multiple times and they worked fine, except test_homescreen_delete_app.py, which fails inttermitently with this error: https://pastebin.mozilla.org/3761783. I will log a bug and try to fix it.