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

The PR aims to convert value pickers family inside system application in to shared building blocks, without affecting the js needed to make it work in system application, in order to being refactorized as part of the Building Blocks refactor project.

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.