Save Responses to File generates bad paths and names

**toogoodtopassup** ([Bug 61118](https://bz.apache.org/bugzilla//show_bug.cgi?id=61118&redirect=false)):
Using the Save Responses to File is really inconsistent to the point that it is unusable.

Using all the default options except where noted and making http requests, here are the outcomes:

Completely blank options - nothing filled in
============================================

Files are placed *in the JMeter bin directory* of the form:
1.html
2.html
etc.

The problem: The JMeter bin directory should never be the current directory for any file interaction - It should always default to the same directory as the current jmx file.

Use '~/' as the filename prefix
============================================

Files are written to *the directory above the directory containing the .jmx file* (its parent-parent directory). These files are named like this:
[parent-parent directory name]1.html
[parent-parent directory name]2.html
etc.

The Problem: Using ~/ is the (somewhat) well advertised way to refer to the directory containing the jmx file, however, it doesn't work and instead targets the directory above that. Really I believe this should target the current user's home directory, but that bridge seems to have been crossed.

Additionally, the file name is messed up in this case, pulling in the name of the JMX file's parent-parent directory as part of the generated filename.

Use '~/output/' as the filename prefix
============================================

Files are written to *the directory containing the .jmx file* (not any sort of /output subdirectory). These files are named like this:
output1.html
output2.html
etc.

The problem: While this does now target the directory containing the JMX file, it incorrectly adds 'output' to the file name rather than attempting to place the files in an /output directory despite the trailing slash.

Use '~/output/output/' as the filename prefix
============================================

Files are written to a /output subdirectory next to the JMX file. These files are named like this:
output1.html
output2.html
etc.

The problem: 'output' is still added to the file name. Additionally, this will fail is a /output subdirectory does not exist - This may be by design but should be documented. It think it would be preferable to create the directory by default.

Note: Using the './' path results in the same behavior as using no filename prefix.
Note: The single quotes in the filename prefix were not actually used.

The propose fix
============================================
* Centralize all path resolution so that behaviors are consistent throughout JMeter (similar problems existing in other components like the CSV DataSet config).
* Use ~/ to refer to the user's home directory
* Use ./ to refer to the current directory, which would always be the same directory as the current JMX file.

In the case of the Save Responses to a File component specifically, there should be a separate directory property to configure where files should be written to so that users don't have to try to cram path into a file name.

Severity: normal
OS: All