Local and system headers files project selector

[KeyWorksRW]

Discussed in https://github.com/KeyWorksRW/wxUiEditor/discussions/1434

^{Originally posted by rossanoparis March 12, 2024} It would be very helpful to add a way based on which the user should be able to add files without using "files selector".

The current file selector, adds files making them relatives to wxUI project path, but in case I have to move a project from a folder to another all #includes directives referred to external libraries become messy, and it is an hard job make them tidy again.

Generally speacking, #include directives for files which belong to a C++ projet can be expressed using their relative paths reffered to wxUI project and that's fine, but for files belonging to external libraries, they are often, in my case always, expressed using relative paths referred to directories defined in IDE tools such as VS, CodeLite, ecc ...

Actually the user is able to get it only typing its own files directly into "local_hdr_includes" or "system_hdr_includes" fields. But as you can imagine, when such files are many, it is not confortable.

As I see it, another button to add a string inserted by the user would be helpful also a resizable dialog would be useful.