app: wallet creation should auto attempt to load wallet config files

[chappjc]

Right now the user can choose the file and it will glean the settings from that, but we need this dialog to automatically load the file from the default location for the particular asset. e.g. ~/.dcrwallet/dcrwallet.conf or ~/.bitcoin/bitcoin.conf

[chappjc]

@buck54321 The feeling is this is an MVP item. Something like the following:

• Click Create Decred Wallet
• frontend tries to load ~/.dcrwallet/dcrwallet.conf
• if it has settings for each of the required boxes (defaults OK), just ask for Wallet Password and App Password and Wallet Name / Account
• if it is lacking a required setting, show the full dialog

So only three things to enter if the conf file has everything: the trading account/walletname and the two passwords.

EDIT: have dexc locate the conf and send the settings to frontend