v3 UI shouldn't open/close pane selector by mouseenter/leave

v3 UI shouldn't open/close pane selector by mouseenter/leave
https://bugs.webkit.org/show_bug.cgi?id=152399

Reviewed by Andreas Kling.

Removed the code to open and close the pane selector by mouseenter and mouseleave
since multiple people have complained about the behavior.

* public/v3/pages/charts-toolbar.js:
(ChartsToolbar): Removed the event listeners.
(ChartsToolbar.prototype._addPane): Don't close the pane selector when adding a new pane
to better support the use case of adding multiple panes.
(ChartsToolbar.cssTemplate): Tweaked CSS.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@194240 268f45cc-cd09-0410-ab3c-d52691b4dbfc