Allow template data merge

[monitor]

After my first few trials with templates, it looks like importing a template overwrites all data in the current set of IPTC data.

I had hoped to be able to have several template for various things, like common places, copyright by year, common data origins etc and then add them as needed, merging new data with any existing data fields.

This could be accomplished by adding a new button or option to the 'Edit IPTC -IIM/XMP' dialog

A separate way to manage these 'partial' templates might also be useful from the 'Tools -> Metadata' menu.
It would bring up the same 'Edit IPTC -IIM/XMP' dialog, but empty. The user would then select the desired template for editing or create a new one.