Adding image widget with drag&drop from Bitmaps section

[prasimix]

Currently, when a new bitmap is added to the page, it is first necessary to add an Image widget (which does not have a defined bitmap) and then in the second step the bitmap is defined. It should be possible to add a new Image widget directly from Bitmaps panel or to immediately ask which bitmap to use while drag&drop Image widget. If there is not a single bitmap in Bitmaps, the option to load the bitmap immediately from disk can be offered. Another possibility is to allow direct bitmap drag&drop from file explorer to the page and create a new image widget in the process.