Store client version on separate version file

[HLXEasy]

The client should store it's version on a separate version file like version.txt. With this file in place we could offer further actions especially after an client update. Some ideas:

• Ask for a backup of wallet.dat either right after start of client or out of the (not yet implemented) installer
• Ask for cleanup of whole data directory

Additionally in case of problems we could ask the user for the value inside version.txt, in example if the client is not starting.

[HLXEasy]

Obsolete at the moment