Add SDF import dialog

This adds a SDF import dialog which allows for molecular
structure and descriptor data to be loaded from .sdf files.

The code was moved from the main window to the SDFImporter
plugin.

Change-Id: I758fbed3876a0eecec45bdf5c4f150a5d20f4397
Merge topic 'sdf-import' into master

3b40a4dd Add SDF import dialog