Themes: Add STYLE_THUMBNAIL to PreviewColumns

Change-Id: Ie18264cbede92220335400428e67735866cd442d
Themes: Add helper methods for creating complete component maps

These methods can be used to apply a theme while replacing any
components not found in the theme with the defaults.

Default components come from the default theme for the system
with any missing components in that default theme coming from HOLO.

Change-Id: Idea55cc684ccc8304b01a991c36b6993f55b8880