A follow-on activity to PR #5061. That PR added scrollbars to the dropdown lists when samples were run in a browser. Earlier changes had broken up a number of categories into multiple entries because the dropdown lists were too long and wound up truncated. With the scrollbars, it makes sense to re-consolidate many of those lists, such as ComplexNumbers, which are all essentially the same test as each other. Some break-ups are retained (e.g. Basic/1/2/3/4/5) because they differ substantially from sample to sample; that's kind of an arbitrary decision, but it will have to do for now. I also renamed some of the members so that like members sort together when listing the directory (e.g. all pivot table samples now start with `pvt`). The sample code is mostly unchanged; only the directory and/or member names are changed.
The download samples were moved to their own new directory, and now represent a complete set of all the write formats that we permit. Studying their code, an improvement for `Helper\Downloader` became evident. It had been used merely to copy an existing file to be downloaded. However, by changing the first parameter of `download` to be either the name of the file directory (the existing usage) *or* a `Writer` object poised to issue `save`, you can use it to download your spreadsheet without having to worry about the headers which the browser requires. It then becomes generally useful beyond being used in samples:
```php
$writer = new WriterType($spreadsheet);
$downloader = new Downloader($writer, 'suggestedFileName.xlsx');
$downloader->download();
```
Fix#4600. String incrementation through the `++` operator is deprecated in Php 8.5. Because we make use of that operator to iterate through columns, we are particularly hard hit by that change - unaddressed, it causes over 2,000 errors in our test suite! It is, fortunately, not as difficult as I feared to correct. Replacing the `++` operator with a call to new method `StringHelper::stringIncrement` in 79 statements scattered over 31 source modules (in src, samples, test, and infra) eliminates all the messages in the test suite. It is possible that others are lurking, but I don't know a systematic way of determining if there are others. We'll stick with this for now, and deal with any others as they show up.
This PR will be applied to the master, release390, and release222 branches. It will not be applied to the release210 or release1291 branches, which will now accept security changes only.
Phpstan hasn't identified any errors in Samples in a long time. But, as it turns out, one particular error is suppressed:
```
Variable $helper might not be defined.
```
This error would be generated by almost all samples, and the errors that it suppresses will become problematic if we move to Level 10. We are not yet committed to doing that, but it is pretty easy to write a script to change the samples so that error no longer happens, and there isn't really any reason to delay doing so. The results of that script constitute this PR.
Dealing mostly with mixed variable type. In a great many cases, problems can be resolved replacing `getValue` with `getValueString` or `getCalculatedValue` with `getCalculatedValueString`.
The samples have become unwieldy when running them from a browser. In particular, the drop-down lists are fixed size with no scrolling, and many of them are now just too large. I have moved all the Calculation samples up a level, and broken several categories (Basic, Chart, DateTime, Engineering, Financial, and Reader) into several pieces.
Convert-Online (now found in the Engineering category) had a number of different problems which are now resolved. It is the only member with any significant code change.
oleibman