Allow styles to be extended

[reduckted]

Fixed #135.

You can now extend the custom Visual Studio-themed styles. This required a few changes:

First, the resource dictionary needed to be compiled like any other XAML file by including it as a Page.

Doing that meant the xmlns attributes for the Microsoft.VisualStudio.Shell.15.0 assembly could no longer be used. This is because in the v14 version of the toolkit, that assembly doesn't exist, and is actually Microsoft.VisualStudio.Shell.14.0. We were previously working around that by loading the XAML file from the embedded resources, doing some string replacement, and then parsing the file as a XAML file. I've solved this problem be creating an internal class called InternalResourceKeys in which we effectively alias the properties from Microsoft.VisualStudio.Shell.xx.0 that we need to use.

For example, VsResourceKeys.TextBoxStyleKey defines the key of the style for the TextBox control that we extend in ThemeResources.xaml (previously accessed as shell:VsResourceKeys.TextBoxStyleKey). The InternalResourceKeys class has a property called VsResourceKeys_TextBoxStyleKey which simply returns the value of VsResourceKeys.TextBoxStyleKey, and in ThemeResources.xaml we now use local:VsResourceKeys_TextBoxStyleKey.

I've also added CustomizedStylesDialog to the demo extension to demonstrate how the styles can be extended. There are two examples:

• One extends the toolkit's TextBox style.
• The other extends Visual Studio's Button style.

You can access that dialog via the Theme Window tool window.

[madskristensen]

Awesome. Thanks