Introduce missing CustomData-attributes of KDBX4 format to allow
storing of plugin data for groups and entries - adopt Metadata to use
the same storage mechanism
Add simple view for CustomData as part of EditWidgetProperties
Tracking of CustomData-Modification using SIGNAL-SLOT update-mechanism
Write duplicate attachments to the binary inner header only once and
skip duplicate entries when reading a KDBX4 file.
This fixes a an attachment mapping problem when an attachment appears
more than once in a database (which occurs frequently when editing attachment
entries and history is turned on)
