[Tracking] ScriptFu widgets finalization

This is a catch-all report for remaining 3.0 issues with ScriptFu and GimpProcedureDialog widgets.

The test case is "Plugin Examples>Example in Scheme" (formerly "Sphere v3" test-sphere-v3.scm) This plugin demonstrates all the argument and widget types i.e. SF- constants for arg types.

I suppose the milestone is 3.0 since these widgets are rarely used and non-string breaking fixes can occur after 3.0rc1.

• The Image Chooser widget is absent . I think one plan was to make the Drawables Chooser parameterizeable, to use the same widget but only let the user choose an image. The old ScriptFu uses GimpImageComboBox, which has flaws but might be OK until something better. GimpImageComboBox could be made into a PropWidget, since it is only used by file-export-pdf.c and that use might even be a relic. SF-IMAGE is for "secondary" images. I don't know of any plugins in the repo that use this.

• The Directory Chooser widget has no sensible default and shows "None". A sensible default would be the user's home directory. (Or Documents directory, but GLib doesn't have that.) This can be fixed in ScriptFu, since it is creating the widget custom. The widget implements the GtkFileChooser interface, which has a setter. When the widget shows no sensible default, and a user neglects to choose a directory, a plugin (e.g. Palette>Export As) fails (having nowhere to write a file.) (SF_DIRNAME)

• The Directory Chooser widget lacks a label. There is a TODO comment in the code, gimpproceduredialog.c line 1628. (SF-DIRNAME)

• The File Chooser widget lacks a label.(SF-FILENAME)

• [*] Obsolete SF-VALUE argument type #8463 (closed)

• A Paths Chooser widget is absent (SF-VECTORS)

• ProcedureConfig throws WARNINGS serializing arguments of type SF-IMAGE, DRAWABLE, VECTORS in certain cases

• Justification of labels is inconsistent, centered or left. Some HIG's say should be right justified.

• Checkboxes seem improperly aligned, should be in right column?