Make SD file create and SD file open accept string value instead of text entry field

[AndyLindsay]

Customer suggestion: Make SD file create and SD file open accept string value instead of text entry field.

NOTE, this would be consistent with one of the most commonly used terminal print blocks.

The default in the string value would still be filename.txt. It's just that it would reside in a string value block instead of the text entry field that is currently used. This would also allow users to dynamically create files by supplying a variable set to a string.

[zfi]

The SD File Open block is currently doing real-time filename validation. This can also be applied to a string input block but within the OnChange event handler instead of the hard-coded string input the block has now.

There is not a RD Create File block in the project. I believe that this is under discussion in Solo #529.