AqBanking

• added missing source file for RDH7 message crypt functions

AqBanking6: Statically link plugins into AqBanking.

- therefore I had to move the plugins folder below "libs" and create the
real AqBanking library (containing the plugins now) in "libs"

BUILD: Create libbackends.la

- libbackends.la contains all backends. The next step will be to link
that library into aqbanking.

BUILD: Condensed file hierarchy prior to withdrawing plugin system.

AqHBCI: Fixed a few bugs in dialogs.

AqBanking: Removed unneeded files.

AqHBCI: Removed libaqhbci.so

- the plugin module no longer exports symbols (excect the plugin factory
symbol)
- libaqhbci no longer needed, since aqhbci-tool's functions were replaced
by the AB_Provider_Control framework

AqHBCI: Removed tool hbcixml.

- didn't use this tools for a few years now, kept it just for
sentimental reasons. No need to port it, the basic functionality
has been implemented in aqhbci-tool a few years back.

Completed moving aqhbci-tool to AH_Provider_Control() framework.

AqBanking6: More work on porting backends and tools. Added AB_Provider_Control().

- AB_Provider_Control() is a variable extension provided by backends which
can be used by e.g. setup tools
- with this mechanism in place we no longer need to provide backend...