symbol plugin does not cache

When I close Scratch and re-open it, the symbols have to be reloaded for each file. We should save this data, at least as long as the tab is left open.

Launchpad Details: [#LP1431087](https://bugs.launchpad.net/bugs/1431087) Daniel Fore - 2015-03-12 00:41:58 +0000

首先定位 symbol 插件以及在重新打开 Scratch 时重新加载 symbol 的代码路径。使用多个打开的文件复现该行为，然后验证只要选项卡保持打开状态，symbol 数据就会被保留，并且不会被不必要地重新加载。

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