Remember first RibbonUI in DefaultRibbon

It should fix another cornercase with `Ribbon_Load` firing only once. 
It helps when `Ribbon_Load` fires once it context of some `ribbonType` (we were remembering it's `RibbonUI` in `RibbonUILookup` with that `ribbonType` key) and then when we're loading another ribbon - `Ribbon_Load` event is not fired again, so we won't have `RibbonUI` for this `ribbonType` in `RibbonUILookup`. The solution is to remember first `RibbonUI` as `DefaultRibbon` and use it when there's no ribbon under specific key in `RibbonUILookup`.

Fixes #44