make the agent picker remember the previous value

Saying you might sometimes accidentally click "yes"
when editing user agent string and then the previous setting
goes away or there is a typo in the customized string
and you want to modify it slightly.

And making the agent picker remember the previous value
solves all the problems above :)
Merge pull request #128 from Javran/agent-picker-memorize

make the agent picker remember the previous value