Allow searching for new line characters

[julen]

Actual new lines cannot be searched in Pootle, or at least the behavior here is unexpected/undefined given the existing issues with backslash (#3941).

Once the editor uses symbols to denote new line characters, this could be extrapolated to the search bar and allow searches to include such symbol, which then would encode to a newline character when submitting. Having help to insert such symbol would be valuable.