Generate symbols list only if the "Source Code Browser" tab is active

[jpfleury]

If I have huge files opened in gedit, navigate through opened documents slows down a lot, because it takes a few seconds for "Source Code Browser" to generate symbols list for each file. However, I don't always need "Source Code Browser". Sometimes, I just have another tab active on the side pane (File Browser, Documents, etc.).

So I think it would be useful to don't automatically generate symbols list when we change the active document. Instead, symbols list could be generated only when we change the active document AND the "Source Code Browser" tab is the active one (the one selected and displayed) on the side pane, or when we click on the "Source Code Browser" tab to make it the active one.

[Quixotix]

Great idea. I'll get to it when I have some more free time. Thanks!