Fix EZP-22313: HTML pasted into literal ends up outside literal tag

JIRA: https://jira.ez.no/browse/EZP-22313

# Description

With Chrome, when trying to paste some code inside a literal, the code is pasted outside of the literal area. This happens because the paste mode is not set to *plain text*, so the serializer creates some paragraphs and tries to fix the code.

The patch makes sure that the paste mode is set to *plain text* when the caret is inside a literal tag. The fix in popup_utils.js is also needed, without it, if the user does not remove the generated non breaking space, the literal area is split in several areas.

# Tests

manual tests in IE9, IE10, Firefox and Chrome