Similar to KeePass2 most elements are checked for a
Protected="True"
attribute. Previously, files KDBX 3.1 files with protected Binary
entries, as seen in the wild, could not be read (mangled passwords and
file attachments).
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)
