ENYO-3287: Do not sync opened files without ACE session when designer mode is toggled on

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-3287

Do not sync file content for file without JS/CSS code like images when designer mode is toggled on

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com