Ensured that style array is cleared when Builder parse tree is reloaded

Ensured that style array is cleared when Builder parse tree is reloaded

If another JSON file is merged into an existing builder instance, it doesn't clear
down the styles array, which means that changes to existing styles aren't picked
up if they have already been parsed.

(In normal use, style sheet is merged before controls are read; or a new application
style sheet clears the existing builder instance).

Change-Id: If5320925e3ca6d7b1587e0c359c05e915ca47390
Signed-off-by: David Steele <david.steele@samsung.com>