[wallpaper/image] Mark webp and tiff as valid image types

They work, but they weren't marked as such. Now you can drad-and-drop
images of these types onto the desktop to get the correct drop menu.

BUG: 426379
FIXED-IN: 5.20
(cherry picked from commit fed19d4322)

wallpapers/image/imagepackage/metadata.desktop

@@ -131,5 +131,5 @@ X-KDE-PluginInfo-Name=org.kde.image
 X-KDE-PluginInfo-Version=
 X-KDE-PluginInfo-Website=https://kde.org/plasma-desktop
 
-MimeType=image/jpeg;image/png;image/svg+xml;image/svg+xml-compressed;image/bmp;
-X-Plasma-DropMimeTypes=image/jpeg,image/png,image/svg+xml,image/svg+xml-compressed,image/bmp
+MimeType=image/jpeg;image/png;image/svg+xml;image/svg+xml-compressed;image/bmp;image/webp;image/tiff;
+X-Plasma-DropMimeTypes=image/jpeg,image/png,image/svg+xml,image/svg+xml-compressed,image/bmp,image/webp,image/tiff

wallpapers/image/slideshowpackage/metadata.desktop

@@ -130,5 +130,5 @@ X-KDE-PluginInfo-Name=org.kde.slideshow
 X-KDE-PluginInfo-Version=
 X-KDE-PluginInfo-Website=https://kde.org/plasma-desktop
 
-MimeType=image/jpeg;image/png;image/svg+xml;image/svg+xml-compressed;image/bmp;
-X-Plasma-DropMimeTypes=image/jpeg,image/png,image/svg+xml,image/svg+xml-compressed,image/bmp
+MimeType=image/jpeg;image/png;image/svg+xml;image/svg+xml-compressed;image/bmp;image/webp;image/tiff;
+X-Plasma-DropMimeTypes=image/jpeg,image/png,image/svg+xml,image/svg+xml-compressed,image/bmp,image/webp,image/tiff