Add an option to scan the sources for read permissions before the actual backup is run

[jose1711]

It may be better to find and fix "file cannot be read - not enough permissions" errors before the actual backup attempt. In shell this can be achieved with:

find $sources -print0 | LANG=C xargs -0 stat 2>&1 | grep 'stat: cannot statx '

This of course can be done by pre-backup command but less-technical users would probably appreciate an optional "check sources" button in Misc tab.

[jose1711]

Also maybe it would be helpful if there's a dedicated env. variable for accessing the source files/directories.