Use one dialog for image export from different SV subviews

Unify images export from different Sequence Views subviews. Currently:

• A single sequence image (zoom view, details view) can be exported using a button on a sequence toolbar.
• A 3D structure can be exported from a context menu.
• A circular view can be exported using a button on the circular view.
• A chromatogram image can't be exported.
• A graph image can't be exported.
• There is no way to export several sequences, several circular views, or several 3D structures.