Add option to open folder recursively

[FLeXyo]

I added the option to open a folder recursively because I needed it for personal use, but I thought that I might as well make a PR for it in case you think it's a good idea.

Potential improvements:

• Add the Microsoft.WindowsAPICodePack-Shell package so we can use CommonOpenFileDialog with the IsFolderPicker property set instead of the awful FolderBrowserDialog. I didn't want to do this without asking since it requires a package to be added.
• Change the "Open file" button to a ToolStripSplitButton and add the recursive option under it. I tried this approach but the dark mode theming didn't seem to work properly with the split button, so I abandoned the idea as I have never worked with Windows Forms before.

I would rebase from master, but the conflicts in the resx files scare me and I haven't worked with Windows Forms before.

[Beelink]

Thank you for the good idea! I will try to implement it.

P.S.
I use self-made library for the dark mode so you almost had no way to implement dark mode by yourself.

[Beelink]

Now it looks like this:

You can check how it works by downloading the binaries from here:
https://github.com/ModuleArt/quick-picture-viewer/tree/master/quick-picture-viewer/bin/Debug