Make PreferencesReaderUtils more generic.

The way we're handling Yaml files ends up with an uncast Object
and we'll need to use these methods whenever handling Yaml.

Move PreferencesReaderUtils into utils, rename to just
YamlReaderUtils, and change the exception types it throws.

Change-Id: I5602740277185f20dab90621f73ed30204711c8b
Reviewed-on: http://gerrit.dmdirc.com/3518
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager