Don't check 'Extraction into subfolder' with single-file archives

The ExtractionDialog only needs to know if it should enable the 'Extraction
into subfolder' groupbox, not why.
We rename setIsSingleFolder() to setExtractToSubfolder() and we call it with
archive->hasMultipleTopLevelEntries() as argument.
This fixes the regression with single-file archives.

CCBUG: 374253

Differential Revision: D3873