ux: placed orders should try to survive Bison wallet restart

[norwnd]

Running Bison wallet on personal PC/laptop presents the following challenge:

• I place a bunch of orders
• after a while I have to shut down (or restart) Bison wallet - be it due to laptop needing an OS update, or just me requiring all the resources of my PC for another task
• when I start Bison wallet next time my orders have been revoked by server (I assume there is also reputation loss) and I have to place them again manually one by one

I could cancel orders manually also before shutting down Bison wallet, but I don't even always remember Bison wallet is running (and that's too much to ask anyway).

This can probably be automated by:

• storing a snapshot of open orders on disk
• cancelling orders upon normal termination of Bison wallet
• restoring orders (if any) from that snapshot upon Bison wallet start