Improve Drag'n drop of report files

[de-jcup]

User feedback:

"DND only works on part of view - would be better to open SecHub report no matter where drop is done"

Hm... the drag and drop support could be improved in different ways:

• We could add DND support on most of the components (as originally wished)
• We could add DND support just for the tree

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^{Albert Tregnaghi albert.tregnaghi@daimler.com, Daimler TSS GmbH, imprint}

[de-jcup]

With last commit, drag and drop of report files do now work in the read marked areas: