Add default value and current value for KeyComboBox

The defaultValue property makes it possible to set the default value to
be used in case that the persistent options dictionary does not have the
requested option key.

The value property is a provided to conveniently query the currently selected
option value for the given KeyComboBox element.
Remove a debugging message
Provide default theme value in the constants dictionary

The default theme is dependent on the current platform as reported by the
device module. If running on the Jolla/Sailfish OS, the silica theme is used,
otherwise the default/classic theme is used.
Add a separate description for the full mode
Use correct default theme

Correctly use the silica theme on Sailfish OS and the classic/default one
everywhere else.