[X] Design time properties

[StephaneDelcroix]

An alternate xaml resource file provider can request the XamlLoader to
not ignore normally ignored properties in prebuilt XF design xmlns, as
in the following snippet:

<ContentPage
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://xamarin.com/schemas/2014/forms/design"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
  <Label d:Text="Bar" Text="Foo" x:Name="label" />
</ContentPage>

The d: should be the prefix used by default for this, but any other
prefix will do too.

The d:Text property maps to the exact same property as Text, as the
XmllnsDefinitionAttributes are identical (that's convenient from a Intelisense
point of view), but, when (and only when) instructed by a provided Xaml resource
loader, the d:Text will override the Text property.

This works with virtually all properties defined on built-in Xamarin.Forms
controls, but it doesn't mean it's a sane idea to try to assign design value
to all existing properties.

The API for setting the ResourceLoader had to change, and instead of taking
pre-defined arguments, it accepts and returns query and response types. This
is slightly less convenient to invoke through reflection, but way more easy
to extend in the future.

As a side note, I'm quite proud, and thankful, of the work done by that 5-people
2-hours brainstorming-meeting that came up with the new name for the now
deprecated ResourceProvider.

VS bug #783304

[mattleibow]

nice 😄

remarks addressed