Support case insensitive image extensions

[CsatiZoltan]

I realized that I couldn't find an image file with .PNG extension. It is because ImagePy only loads .png (i.e. with lower case). You could consider accepting upper case extensions too.

[yxdragon]

I had tested it in deepin linux, and I both png and PNG is OK, what's your os?

[CsatiZoltan]

If I use File -> Open, then only the images with lowercase extensions are listed. You can check it by investigating the fifth element of the list ls here: https://github.com/Image-Py/imagepy/blob/9710a277ad5711ca91c19c9324cb1bc18f4e2ff6/imagepy/core/manager/iomanager.py#L16 To me, it shows

dict_keys(['jpg', 'jpeg', 'dat', 'dcm', 'npy', 'tif', 'tiff', 'mat', 'png', 'gif', 'bmp'])

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On the other hand, File -> PNG -> PNG Open gives 'PNG files (*.PNG)|*.PNG' in the filt variable: https://github.com/Image-Py/imagepy/blob/9710a277ad5711ca91c19c9324cb1bc18f4e2ff6/imagepy/core/util/fileio.py#L47 resulting in (see the red highlighting): I don't think it matters, but I work on Ubuntu 18.04 with Gnome 3 desktop.