spinner: add label format validator

spinner: add label format validator

Summary:
Check for all error conditions case in elm_spinner_label_format_set
and set label format only if its valid.

Signed-off-by: Shilpa Singh <shilpa.singh@samsung.com>
@feature

Test Plan:
1. Launch elementary test spinner
2. Set invalid formats e.g. formats with out %, formats with more than
one format specifier "%d %s" etc:-

Reviewers: jpeg, cedric

Reviewed By: cedric

Subscribers: govi, buds, subodh6129

Differential Revision: https://phab.enlightenment.org/D3720

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>