Add HTTP webserver feature (for testing only at the moment)
Add experimental webserver API and home page

Added webserver::IRequest/IResponse/IDelegate interfaces.
Add InkType configuration for tools
Support InkType tool setting in ToolLoopImpl
Move tools/ink_type.h to settings/
Add InkType::kEraseInk
Hide ink combobox for tools that doesn't make sense to change the ink

For example, eyedropper, selection, etc. are special tools that shouldn't
be allowed to change the ink (to avoid confusion).