Properties filter: Optionally support Java escaping of colon, #, =, !

Per https://docs.oracle.com/javase/7/docs/api/java/util/Properties.html#store%28java.io.Writer,%20java.lang.String%29

Then every entry in this Properties table is written out, one per line. For each entry the key string is written, then an ASCII =, then the associated element string. For the key, all space characters are written with a preceding \ character. For the element, leading space characters, but not embedded or trailing space characters, are written with a preceding \ character. The key and element characters #, !, =, and : are written with a preceding backslash to ensure that they are properly loaded.

The filter doesn’t seem to handle this – it will extract \: as \: rather than :.

We should add an option that, when present, causes \ + :#!= to be treated as an escape on extraction, and to be re-inserted on merge.