Convert dropzone widget into ImGui widget

[mwestphal]

Context
F3D has a "dropzone" to let user know they can drop files into it. This dropzone is implemented using VTK actors, but it should be converted to ImGui widgets, especially because this is the last vtkTextActor in use.

The dotted line itself can stay a vtkActor if needed, only the text part must be converted, but converting all is of course perfectly fine.

[Ni-g-3l]

Hello ! I try to resolve this issue here : #1990

However, how can I update tests in order to match the new dropzone ?

[mwestphal]

Discussion happened on discord, lets try to implement it so it looks like that: