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:

imagepy/imagepy/core/manager/iomanager.py

Line 16 in 9710a27

ls = [cls.reader[i].keys() for i in cls.reader.keys()]

To me, it shows

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

---

On the other hand, File -> PNG -> PNG Open gives 'PNG files (*.PNG)|*.PNG' in the filt variable:

imagepy/imagepy/core/util/fileio.py

Line 47 in 9710a27

filt = '|'.join(['%s files (*.%s)|*.%s'%(i.upper(),i,i) for i in self.filt])

resulting in (see the red highlighting):

I don't think it matters, but I work on Ubuntu 18.04 with Gnome 3 desktop.