Signal when image is renamed from Lua script

[wpferguson]

raise metadata changed signal when image is renamed so that lighttable thumbnail will reflect the name change.

Fixes darktable-org/lua-scripts#598

[wpferguson]

@TurboGit to test:

• start rename images script
• select an image in lighttable
• go to rename images and use pattern $(FILE.FOLDER)/$(IMAGE.ID).$(FILE.EXTENSION)
• click rename
• hover the image to see the changed filename

[TurboGit]

Thanks!

[anoderay]

This would go nicely with darktable-org/lua-scripts#712 @wpferguson :-)