Add a global select/deselect checkbox

[brandelune]

Is your feature request related to a problem? Please describe.
When the number of files excedes one screenful, it is not practical to deselect many files.
When one wants to select a small number of files in a long list, it is not practical to deselect the other files.

Describe the solution you'd like
A checkbox in the list header that either selects or deselects all the files.