Add X-button for close to header of the right side panel

The component properties editor has a X-button in the header that closes the side panel. The same button must be added to the content editor and create content step 1 and 2.

For the content editor:

• add the X button at top: when clicked, the same action is taken as when the current Close button is clicked.
• rename the "Close" button at the bottom to "Discard". Button is active only in case of unsaved changes.
  Question message 'Discard unsaved changes to '<Documentname>'?' should appear.
  On choosing Discard in the question message the changes are discarded, the sidepanel remains open (and is refreshed).

For create content step 1:

• add the X button at the top: when clicked, close the editor, even if there are field changes

For create content step 2:

• add the X button at the top: when clicked, the same action is taken as when the Discard button is clicked. The button below will be removed. See the linked issue CHANNELMGR-2174 for a follow up. This is discussed with cdekort.

For Open UI:

• the close button at the bottom can be removed