[NuGet] Install NuGet packages referenced by project templates.

Project templates can reference NuGet packages by adding the following xml
inside the Project section of the template:

    <Packages>
        <Package Id="NUnit" Version="2.6.3" />
    </Packages>

NuGet packages are installed in the background after the project is created.

Currently NuGet packages can only be installed from the configured NuGet feeds.
[NuGet] Allow addins to define local package repositories.

Using an extension an addin can define a directory where NuGet packages
used by project templates can be found.

  <Extension path="/MonoDevelop/Ide/ProjectTemplatePackageRepositories">
    <PackageRepository path="packages" />
  </Extension>
[NuGet] Rework project templates with NuGet packages.

Determine projects and associated NuGet package references in
project template classes.
Make ProjectTemplatePackageInstaller abstract.
[NuGet] Create project template package install info on UI thread.

Get all the information required from the project model on the UI
thread and then install NuGet packages on a background thread.

Still need to review the code that updates the project as a NuGet
package is installed.
[NuGet] Show project template errors in Package Console pad.

When a NuGet package fails to install into a new project then an
error is shown in the status bar and the Package Console pad.
The Package Console pad can be opened from the status bar.
[NuGet] Show NuGet output in Package Console when creating a project.
[NuGet] Show progress when installing NuGet packages in templates.

Update the progress by one step in the status bar after each NuGet
package is installed from a project template.
Add dialog class with a merged title/tool bar

Based on the extended title bar window class.

This Xwt.Dialog subclass adds a 'header' section which blends with
the title bar when possible. Added a default implementation and
a Mac-specific implementation.
[NuGet] Add basic dialog to test extended title bar dialog.
[NuGet] Show packages in Add Packages dialog.

Search not working.
Wrong search control being used.
Package installation not currently being done in background.
List view text does not word wrap.
Package information resizes on selecting a different package.
Can only see first page of packages.
[NuGet] Added basic search as you type in Add Packages dialog.

Needs to be updated to use custom search control instead of
text entry control.
Merge branch 'nuget-project-template' into nuget-extended-title-bar-dialog

Conflicts:
	main/src/addins/MonoDevelop.PackageManagement/MonoDevelop.PackageManagement.csproj
	main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj
[NuGet] Extract background package installer for re-use.
[NuGet] Fix Add Packages dialog exception when closed immediately.
[NuGet] Install package from Add Packages dialog in the background.

Close dialog when Add Package button is clicked and install
the NuGet package in the background.
[NuGet] Allow installing multiple packages from Add Packages dialog.

Add check boxes to Add Packages dialog.
Currently only installs the packages selected and displayed.
[NuGet] Show aggregate source as 'All Sources' in Add Packages dialog.
[NuGet] Show NuGet Packages in Solution pad.
[NuGet] Update NuGet packages in Solution pad when packages installed.
[NuGet] Update status bar when restoring packages.

Use the same Package Console pad for restoring and installing NuGet
packages.