Add programming interface for modal dialogs

[mhjlam]

Greetings! I noticed that nfd does not currently support modal dialogs; windows that force the user to interact with it before returning to a parent application. As I needed this behavior I have adapted the code of this library rather than write the whole thing myself. Perhaps the changes are useful for others as well.

Note that I have only tested this on Windows 10. The changes made for the GTK version should be correct as well, looking at the API documentation. Furthermore, since I have no experience with Cocoa, I am uncertain how modals work on OSX, but a quick glance at the API suggests that calling function runModal should already open a modal window as is in the current version.

[mlabbe]

Thanks for sharing your contribution. An acceptable pull request would meet the guidelines in the submitting pull requests.md: https://github.com/mlabbe/nativefiledialog/blob/master/docs/submitting_pull_requests.md which highlights that the code has to be fully tested on all platforms. Testing on all platforms is hard which is why the api implementer has to do it; it spares the programmers.

In an acceptable PR, I would also avoid doubling the API surface area, and consider a function to enable or disable modality globally as an alternative.

I think the idea of modal dialogs is useful and I hope a pull request that meets the standards laid out comes up eventually.