libkeepass rewrite

[Evidlo]

@pschmitt @crass

I've been working on a complete rewrite to add support for KDBX4 (and KDBX3) and shrink down the codebase.

Things to note:

• only tested on python3
• no twofish support yet
• no entry unprotecting yet fixed
• kdbx4 seems to use some new binary format for storing dates? all my date fields look similar to 1M100A4AAAA= fixed in pykeepass
  edit: see here

Also, getting pykeepass working with it was pretty trivial. See the libkeepass_proto branch:
https://github.com/Evidlo/pykeepass

Let me know what you think.

[Evidlo]

Should protected/unprotected opening even be configurable? Right now it always opens unprotected. We can't have any memory security with python anyway.

[Evidlo]

@pschmitt How do you feel about possibly merging pykeepass and libkeepass? I think there's no practical reason for the codebases to be separate anymore, and anyone who needs low-level database manipulation still has access to PyKeePass.kdb object.

Also @firecat53 would probably be interested in this thread.