JBIDE-17472 Unable to edit preview text in CSS Preview view.

Updated "Double click to edit preview" to "This is a Preview Text"
JBIDE-17169 HTML5 Palette templates should be available for content assist by key words

Added keywords to Mobile Palette items. Modified xml tag completion
proposal computer to apply keyword when cursor is within a text node
to proposals and set high relevance to those having the keyword.
Tests are added.