Creating a file while having a file or directory selected will now
create the file under the selected directory, or in same directory as
a selected file.
Creating directories works the same way.
Right click -> Open on selected files will now open the selected files.
Deleting selected files will now delete the selected files.
As suggested by sagefarrenholz this was accomplished by adding a private
function HackStudioWidget::selected_file_paths() that returns a
Vector<String> of the current project tree selection.
Function HackStudioWidget::selected_file_names() is removed due to not being used.
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