add menu item "Navigation" -> "Scroll from source"

Actually, the only reason the item was added - to get an ability to assign keyboard shortcut on it.
Use case follows: I have a project with huge source tree. And I want to see files in the same folder for source file I edit. Navigation project tree is way simpler than using navigation bar (with Alt+Home). And I should move mouse twice - first to icon 'scroll to source' in pane, and the second to files in tree.
With this change one may assign shortcut on 'scroll to source' (or whatever you name it), to save some time.

Below are 2 shots describing changes
![what-we-have](https://f.cloud.github.com/assets/1007300/1692432/5acc1a64-5e7b-11e3-8468-e7435effa68d.png)
![what-we-will](https://f.cloud.github.com/assets/1007300/1692433/5b0b2a42-5e7b-11e3-9d9e-a97063a6af46.png)

ps. I added support for this menu item in project view and groovy mvc project view both. Luckly without any troubles.
pps. maybe it's a good ide to disable action (menu item) while scroll-from-source providers won't response to action.