Reset progress bar text when directory loading starts

Dolphin shows a different text for whether it is loading the directory or currently sorting its contents.
However, when entering a folder that causes sort criteria to change (e.g. because of different view properties in that folder),
the status bar text is changed to "Sorting..." while it is still actually loading the folder.
When loading a directory starts, reset any status bar text to ensure "Loading..." is displayed in this case.

Differential Revision: >https://phabricator.kde.org/D23459