clear log button and show/hide volume checkbox added

[codeling]

In general, this looks very good 👍 , thanks for the contribution!

With the display of the volume, there are two issues (though in my opinion these are very minor and don't require immediate fixing):

• The 3D render window does not update automatically after the checkbox state change; one has to for example move the mouse into it to trigger the update. Fixing that is not straightforward unfortunately. It requires calling the Render method of the vtkRenderWindow that the volume is rendered in (which the iAVolumeRenderer does not have access to directly at the moment). So either dlg_modalities or iAVolumeRenderer would need to get passed a pointer to the vtkRenderWindow they should update; or they would need to emit a signal which e.g. the MdiChild listens to, and then calls the vtkRenderWindow::Render method.
• When a modality is selected and the "Edit" button right of the list is pressed, a dialog opens. There is also a "Main Renderer" checkbox in this dialog, that has the same functionality as the outside checkbox; but the two checkboxes are not synchronized. Ideally I guess the one in the dialog should be completely removed to avoid confusion.

[k-lara]

The updating should have been implemented but i think that i forgot to commit those changes, because I added this later on and did not commit anymore.

[codeling]

Then it should be just a matter of commiting and pushing, it should then automatically show up here. Let me know if you need any assistance!

[codeling]

This seems to be a duplicate of the other, already merged pull request to develop branch, closing.