Fix issue that wrong directory is shown

Commit 680009b522b2c9eda03201ac02dc18994b5a8bd0 resulted in a regression when switching from a view in the column-mode to a view in the icon- or details-mode: The wrong content has been shown. This patch reverts commit 680009b522b2c9eda03201ac02dc18994b5a8bd0 and solves the performance-issue in combination with previews by triggering the preview-generation in a queued way.