Don't accept DNG or any raw files using Add Images window

[Bicyclops]

Hello, I'm Morgan from RawTherapee.

Testing Hugin_Panorama_Creator-git-20181103-x86_64.AppImage for its integration with RawTherapee. I'll open separate issues for things I notice.

Edit > Add Images, Hugin accepted raw files as input. Some raw file formats such as Canon's CR2 crash Hugin, some formats such as Fujifilm's RAF are rejected, while other formats such as NEF and DNG are opened and their embedded thumbnails are used.

This is wrong, and there are two possible solutions:
1. Do not accept any raw files in the Edit > Add Images dialog.
2. Do accept raw images in the Edit > Add Images dialog, but when the user clicks "Open" then automatically switch over to the "Import raw files" window.

[Bicyclops (bugs-8)]

Screenshot showing that when I opened DNG files using Edit > Add Image, only the small embedded thumbnail was used.

[tmodes (tmodes)]

By default with Add images only image files are shown. Only when you select All files you will see the raw files.

Here we are using vigra identification of image file. Some raw file are here rejected, but some will accepted, because technical these files are TIFF files.
I struggling to remove the all file filter. Then somebody will complain why can you not select e.g. a file with mixed capitalization in the extension.

[Bicyclops (bugs-8)]

Confirmed that the default filter setting, "All image files", does not show raw files.

P.S. The capitalization here is wrong, it currently is "All Image files", should be "All image files".