WIKI-767: [IE8] When pointer is focusing on text field in Rich Text then...

... switch to source mode, text field is uneditable

Problem analysis:
- When the pointer is focusing on an invisible object in IE, it cannot be focused in other field.
- After loading rich text field, this field is focused. But in the source mode, I cannot see this behavior.

Fix description:
- Force forcusing on source field in Source mode