Special Characters in Image File Name

[oleibman]

Fix #2415. Fix #1470. If path name of image contains anything other than ASCII, or if it contains # or space or probably other exceptions, PhpSpreadsheet creates a file that Excel cannot, for whatever reason, read (it is valid xml). When adding an image to a spreadsheet, Excel does not retain the original path name; PhpSpreadsheet does, but probably shouldn't. It is changed to save the image file in the zip as the MD5 hash of the original path name. This produces a file that Excel can read. In addition, it ensures that, if the image is used in multiple places, it is saved in the Excel file only once. As a consequence of that change ... Fix #2323. Fix #1153. Fix #543.

Because this error becomes evident only when opening the file in Excel, it is difficult to write a test case. I have instead duplicated sample Basic/05... using image files whose names match the reported error conditions.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?