Use Last Wallet as Default

Added fourth option to startup wizard to allow the user to select an existing wallet file.  It's possible this file exists somewhere else on the file system (eg, maybe from another installation, or as a backup, etc.).  So we should allow the user to choose this existing wallet file (instead of forcing them to name it "default_wallet" and place it in the expected folder).

Second commit is that Electrum will now try to open the last wallet file that was used (instead of always looking for one named "default_wallet" on startup).  Some users (like myself) may have several wallet files they alternate between, so it doesn't make sense to just use "default_wallet" every time it starts.  Plus, users may want to rename their wallet file to something other than "default_wallet".