Rehaul asset importing

Content library buttons now launch QFileDialog instead of a separate
explorer. Assets imported via content library buttons are sorted to
default asset directories if those directories exist. Otherwise the
assets go into the selected folder.
A new Import Asset button was added for importing any asset.
Import Asset context menu option was added. It is equivalent to
dragging assets to selected row.
Import Asset file menu option was added. It is equivalent to clicking
Import Asset button.
Dragging assets to root folder or outside the folder structure on
project view autosorts assets to default directories. Otherwise
dragged assets go to directories they were dragged to.

Task-number: QT3DS-339
Change-Id: Ifb9290f57564364d01590819116645952fd5f7f5
Reviewed-by: Marianne Yrjänä <marianne.yrjana@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

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