Screen capture unique file names and location

Summary: Screen captures now have unique names, so they don't get overwritten. By default they are saved to the desktop, but the path can be overwritten by setting `screenCapturePath` in `~/.sonar/config.js`

Reviewed By: jknoxville

Differential Revision: D9120822

fbshipit-source-id: ab6880eac475da3839f08c6e644c16bdc8693647

src/ui/components/Button.js

@@ -163,7 +163,7 @@ type Props = {
   /**
    * onClick handler.
    */
-  onClick?: (event: SyntheticMouseEvent<>) => void,
+  onClick?: (event: SyntheticMouseEvent<>) => any,
   /**
    * Whether this button is disabled.
    */