Import wizard allows recursive import of directory

Selecting sparse files (old behaviour) is still possible by unchecking the
checkbox. The recursive behaviour seems like the most common use case so the
option is checked by default.

Changes in this commit:

* rename the 'include subdirectories' option to 'recursive directory import'
* when checked, the tree view shows only directories
* when checked, only a single item can be selected. The file counter counts all
files contained in the selected directory (recursively).
* the 'select all' button has been removed (didn't work, don't see a use case
for it)