i3bar: don't reset verbosity when parsing config values

i3bar: don't reset verbosity when parsing config values

When i3bar is called with the -V flag but there is no 'verbose yes'
directive in the bar {} config, the verbosity config value is reset.

This will introduce the opposite, negligible issue: you can't disable
i3bar's verbosity by deleting the 'verbose yes' directive in the bar {}
config. To fix this we would need an enum for config.verbose.

Closes #3220.