Clear up dialog title in add contentlet

[fmontes]

As a user I want to know correctly which contentlet I'm about to edit

Expected Behavior

When user open a dialog to edit a content the dialog title should show blank until the title is loaded

Current Behavior

Is showing the previous contentlet title

Possible Solution

Is the header attr here

Known Workarounds

N/A

Steps to Reproduce (for bugs)

1. Go to Content > Search
2. Click ... > add content
3. Close the dialog
4. Try to create another content but a different type

Acceptance Criteria

1. When user open the dialog to create a content it should always show blank until right dialog title loads.
2. Work in all the supported browsers (don't forget IE11 and iPad)
3. Multilanguage
4. Unit test

[bryanboza]

Fixed, tested on master // Postgres // FF

[bryanboza]

Fixed, tested on release 5.0.2 and works as expected