[Ide] Add support for error bubbles in the solution pad

Reorganize ExtensibleTreeView, so that instead of storing node information
in separate columns we store all of it in a single column of type NodeInfo.
This will make it easier to add more formatting options.
[Ide] Improve display of errors in solution pad

Added StatusSeverity method to NodeInfo and PopoverTooltip. It can be used
to show standard information or error messages.
[Icons] [MonoDevelop.Ide] New solution pad icons
See https://github.com/xamarin/design/issues/212
[Ide] Adjust style of status icons in the solution pad
Merge branch 'new-solution-pad-errors'