[Core] Fix potential NRE
[NuGet] Add caching for package icons.

Do not make multiple http requests for the same package icon.
[NuGet] Fix removed package source used in Add Packages dialog.

Removing or disabling a package source in Preferences when it was the
active package source was still being used in the Add Packages dialog.
The combo box was empty but the old source was still being used.

Handle all package sources being disabled. The Add Packages dialog
will only show Configure Sources in the combo box.

Handle all package sources except one being disabled when the active
package source is the Aggregate source (All).
[Ide] Welcome page buttons are now more flexible.
[NuGet] Fix templates with NuGet packages not finding id/version.
Merge remote-tracking branch 'origin/master' into retina

Conflicts:
	main/src/core/Mono.Texteditor/Mono.TextEditor.csproj
Bump md-addins
[NuGet] Fix file changed warning when adding/removing MSBuild import.
Updated reference to xwt
Updated references to xwt, md-addins
Merge branch 'retina-nuget-add-msbuild-import' into retina
[NuGet] Allow searching for packages by id, tags and description.

Can now run searches for NuGet packages using 'id', 'tags' and
'description' on repositories that support this feature such as the
main NuGet feed.

Example searches:

id:nunit
tags:fsharp
description:mono
[NuGet] Execute all project operations on GUI thread.
[NuGet] Handle errors when packages.config file cannot be read.

Fix unhandled exception when updating packages when packages.config
is invalid.

Fix unhandled exceptions in Add Packages dialog when packages.config
is invalid.

Show error message in Add Packages dialog when the packages.config
file cannot be read.
[NuGet] Fix icon for Package Console pad not being displayed.
[Ide] Fix error when opening reference

Fixes 17784 - XS throws exception on clicking "System.Web.Entity"
reference file, which is missing
Updated reference to md-addins
Bump fsharpbinding
Merge remote-tracking branch 'origin/master' into fsharp

Conflicts:
	version-checks
Merge remote-tracking branch 'origin/master' into retina

Conflicts:
	version-checks