Bug 870747 - Move value selector BB files out of system app

Main tasks achieved by this PR:

* Splited and isolated rules from `time_picker.css`, `spin_date_picker.css` and `bb/value_selector.css` in to 3 independent files.
* Removed `id` dependant selectors and hierachy anchored to `[role=dialog]` and `[data-type=x-selector]` (as used in other Building Blocks)
* That files has been moved & renamed to shared/style_unstable as `time_selector.css`, `date_selector.css` and `value_selector.css`
* Due to selectors conflicts made by `modal_dialog/prompt.css` i had to add a classname to differenciate from system custom dialogs from building blocks dialogs.
* As i didn't want to perform any js changes, i've tried just to have strong&open enought selectors to match any markup wrapper.