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

Start by locating the symbol plugin and the code path that reloads symbols when Scratch is reopened. Reproduce the behavior with multiple open files, then verify that symbol data is retained while a tab remains open and is not reloaded unnecessarily.

Written by the indexing model from the issue text.