Add tests for Watermark Image + Fix Heigh / Width for headers/footers

[PrzemyslawKlys]

When using:

public static void Watermark_SampleImage1(string folderPath, bool openWord) {
    Console.WriteLine("[*] Creating standard document with Watermark Image 1");
    var imagePaths = System.IO.Path.Combine(System.IO.Directory.GetCurrentDirectory(), "Images");
    string filePath = System.IO.Path.Combine(folderPath, "Basic Document with Watermark Image 1.docx");

    using (WordDocument document = WordDocument.Create(filePath)) {
        document.AddParagraph("Section 0");
        document.AddHeadersAndFooters();

        var imagePathToAdd = System.IO.Path.Combine(imagePaths, "PrzemyslawKlysAndKulkozaurr.jpg");
        var watermark = document.Sections[0].Header.Default.AddWatermark(WordWatermarkStyle.Image, imagePathToAdd);

        // TODO: Add handling for watermark image
        Console.WriteLine(watermark.Height);
        Console.WriteLine(watermark.Width);

        document.Save(false);
    }

    using (WordDocument document = WordDocument.Load(filePath)) {

        document.Save(openWord);
    }
}

The console lines throw an error since it only expects picture in document not headers/footers.

https://github.com/EvotecIT/OfficeIMO/blob/850f61707ddf23af1ea2f36e301c264ad3197e47/OfficeIMO.Examples/Word/Watermark/Watermark.SampleImage1.cs#L26-L27