fixed: write_image_file failed with --export with relative path, because the is-relative-to-target-dir check was wrong

Fix #67: remember the value of FilterCtrls when the control is destroyed, and set it again when the control is recreated

  condition ? wxString(something) : wxEmptyString
doesn't work, because wxEmptyString is a wxChar*, and g++ doesn't see the conversion.

TODO: move this function to another file

testset: ignore locale errors (except in a single testcase for them), otherwise all tests fail when there is an error in the locale

tests: consistent format for output, include some information on the machine running the tests, script for testing at top-level

added numeric escapes