Add public constants for values used in default TinyMceSettings

Description

This change makes overriding the toolbar settings easier in cases where the default toolbar doesn't need to change but additional toolbars should be added. For example:

config.Default()
    .Toolbar(DefaultValues.Toolbar, "styleselect");

Or, if a partner wants to configure an empty settings object and needs to load the episerver plugins. For example:

config.Empty()
    .Plugins(DefaultValues.EpiserverPlugins);