[NuGet] Fixed Bug 18951 - Incorrect path separator used for Import.

https://bugzilla.xamarin.com/show_bug.cgi?id=18951

Incorrect path separator used when adding MSBuild .targets/.props
file to a project when installing a NuGet package. On the Mac the
forward slash character was being used for the path to the .targets
file. All paths in MSBuild project files should use backslash on
all platforms. If forward clashes are used then updating a NuGet
package on Windows will fail to remove the old import from the
project. It will also cause the build to fail since the path is
resolved to a directory inside Program Files.
[NuGet] Remove forward slash imports when updating a package.

Fixes Bug 20610 - Xamarin Forms samples will not build on Windows
even after package update.
https://bugzilla.xamarin.com/show_bug.cgi?id=20610

On uninstalling or updating a NuGet package, .targets file imports
are removed from the project if they contain forward slashes and if
they contain backslashes. This allows the project to fix itself
without requiring manual editing of the MSBuild project file.
Fix  20533 - Project Reference 'properties' is showing disabled
Fixed Bug 6620 - Cannot properly remove item from current solution

Bug happened because after deletion file, the solution was not save.
Merge pull request #593 from vitorelli/master

Fixed Bug 6620 - Cannot properly remove item from current solution
[MSBuild] Add extension point for global properties

Also unifies setting core global properties.
[MSBuild] Remote builder support for arbitrary target/item evaluation

Not exposed yet, but used for reference evaluation.
[Core] Fix recursive includes for custom MSBuild frameworks
[Core] Filter overridden assemblies from chains of framework includes

And fix up refreshing/reloading stuff as appropriate.
[Ide] Support MSBuild custom tools
[AspNet] Massive overhaul

* Merged ASP.NET and ASP.NET MVC addins
  - Added support for ASP.NET MVC in Web App
    projects, needed for ASP.MVC 5 projects
* Cleaned and reorganized the codebase
* Moved ASP.NET unit tests into separate assembly
  - Fixed some minor issues in Razor tests
* Removed automatic translation of C# project/file
  templates to other languages
  - Only C# templates are included
* Removed build-time updating of Web Forms CodeBehind
  - Update-on-save is still supported
* Removed unmaintained remote deployment system
  - It was never supported on Windows/Mac
  - Removes dependency on Linux deployment addin
* Updated ASP.NET MVC templates to ASP.NET MVC 5
  - ASP.NET MVC reference is now a NuGet reference
* Removed incomplete default doctype abstraction
  * All template used HTML5 doctype
* Stetic is no longer used for UI
[XmlEditor] Massive cleanup

* Rename public types/namespace to be more meaningful
* Port old tests to new parser
* Reorganize namespaces/directories
* Remove a lot of dead code
* Less stetic