Add more information to Symbol plugin tooltips

[danirabbit]

Problem

The symbol plugin tooltips aren't super useful currently. They just show the name of the symbol which is very redundant

Proposal

Tooltips should show as much information as possible for that symbol to help identify it. For example what type of symbol it is and any code comment associated with that symbol

Prior Art (Optional)

Check out the tooltips in the sidebar on Valadoc.org: https://valadoc.org/granite-7/Granite.SettingsPage.html