Allow explicit loading of hidden files.

This will allow explicit loading of hidden files, but still will ignore hidden files and folders on deeper levels if the "show_hidden_files" option is disabled.

This fixes PICARD-589 and improves PICARD-528.
Separate the file browser show hidden files option from the option to ignore hidden files on load.
Updated news regarding fixed PICARD-589
Fixed Win32 hidden files detection.
Merge pull request #370 from phw/better_hidden_file_handling

PICARD-589: Better hidden file handling