EogExifDetails: Rename to EogMetadataDetails

The widget is not specific to Exif, and is used for XMP as well.

https://bugzilla.gnome.org/show_bug.cgi?id=509406
EogMetadataDetails: Don't install header file

This is not really useful for plugins.
EogPropertiesDialog: Separate Exif/XMP-specific widgets from generic ones

This is just renaming a few widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=509406
EogPropertiesDialog: Make the details view use all available space