Select current directory if no folder is selected

Here I correct a problem that has been annoying me for quite some time.
When selecting a folder to add/import, if no folder was highlighted, we
got the `sif.folderMustBeSelected` error.

Now, we just add/import the current directory if no folder is
highlighted.
Merge pull request #681 from agouge/fix-sif-folder-selection

Select current directory if no folder is selected [SIF]
Here I correct a problem that has been annoying me for quite some time.
When selecting a folder to add/import, if no folder was highlighted, we
got the `sif.folderMustBeSelected` error.

Now, we just add/import the current directory if no folder is
highlighted.