Doc: Added the missing example snippets

The snippet files referred were outside the scope exampledirs for
QtCore. I made of a copy of those files in the snippets folder to
fix this issue.

Task-number: QTBUG-29755
Change-Id: I2f765104394071d035e9cee610945ae290033deb
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Fix Qt Designer warning about being unable to handle property.

Qt Designer warns:

"The property "document" of type 1053 (user type)  is not supported yet!"

when loading forms containing a QTextEdit.

Introduced by 468c22f6732b5755b68f658dbd7a9c4e9e9601d1 .

Change-Id: Ia1b7bdc9f7188e0b092c010056fcd12c9e891fd7
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>