* Adding annotations to backend
* Auto fetching Annotations on the backend
* Closing the loop
* Adding missing files
* annotation layers UI
for https://github.com/apache/incubator-superset/issues/3502
* a few fixes per code review.
- add annotation input sanity check before add and before update.
- make SelectAsyncControl component statelesis, and generic
- add annotation description in d3 tool tip
- use less variable to replace hard-coded color
* 1. after user make sql query and visualize, allow user click title to update slice title, and create a new slice at the same time.
2. don't save new title if it is empty. Will still show old title.
* change saveSlice call response and update explore view
The left panel of the explore view has become crowded and overwhelming
overtime. This PR adds functionality to collapse the control sections,
and sets most sections to be collapse by default as the explore view
opens up.
* breakdown `Query` section for most viz
* bring filters to the top, under Query section
* collapse most sections by default
* removed confusing outdated description for Filter section
* Allow for multiple color schemes
1. create ColorSchemeControl component
2. using the same new control component for linear colors
* add color spectum for linear color scheme
* remove dup css
* fix controls setting for linear color scheme
* minor fix by code review comment
