Fix escape handler in Automation Properties loader

When using org.nuxeo.ecm.automation.core.util.Properties#loadProperties, all lines finishing by backslash "\" will escape the line to append the next line. No matter the number of these. (if escaped or not)

Use case:

• I use "Document.Update" (Update Properties) Automation operation
• In my "properties" parameter I put these values:

  dc:title = test \
  dc:description = description
  

• I get

  dc:title = test dc:description = description
  

• I should have

  dc:title = test \
  

• And If I put:

  dc:title = test \\
  dc:description = description
  

• I get the same result (what ever the number of )

Proposal:
We activate or not by system property this part of code in org.nuxeo.ecm.automation.core.util.Properties#loadProperties:

if (line.endsWith("\\")) {
                line = line.substring(0, line.length() - 1);
                prevLine = prevLine != null ? prevLine + line : line;
                line = in.readLine();
                continue;
}

Warning:
This method is used by workflow, automation and can have lots of impact on existing implementations.

cf: See SUPNXP ticket for this use case feedback.