common: Fix BufferViewConfig upgrade defaults

Fix initialization of default values when upgrading BufferViewConfig.
This solves the issue of showSearch() being set to true on upgrade
despite having the default of false.

Search doesn't usually need shown as the activate shortcut will make
it temporarily appear.

Details: When loading a buffer from configuration, the
"fromVariantMap(properties);" path was taken.  As this side lacked
the initialization, defaults weren't set for newly-added values.

Thankfully, it really is just this simple (fingers crossed).

Add some documentation love, too.