Made a number of changes aimed at reconciling cell comments and form controls in keep_vba mode.

Created originally on Bitbucket by jbovey (John Bovey)

Was already merged in Bitbucket before import, marked as merged by the import user

In particular, if a worksheet has a legacyDrawing link to a vml file then the vml parts of any new comments are merged into the existing vml file rather than being written out in a new vml file.

There are also a few changes to names of variables and links to reflect the fact that a worksheet has a single vml file containing descriptions of controls and comments.