Fix #51 - Create VS2012 solution file when needed.

src/Main/Base/Project/Project/TargetFrameworks/DotNet4x.cs

@@ -94,6 +94,15 @@ namespace ICSharpCode.SharpDevelop.Project
 			// This implementation is used for all non-client versions of .NET 4.x
 			return fx != null && fx.IsDesktopFramework && fx.Version <= this.Version;
 		}
+		
+		public override SolutionFormatVersion MinimumSolutionVersion {
+			get {
+				if (this.Version <= Versions.V4_0)
+					return SolutionFormatVersion.VS2010;
+				else
+					return SolutionFormatVersion.VS2012;
+			}
+		}
 	}
 	
 	/// <summary>

src/Main/Base/Project/Project/TargetFrameworks/TargetFramework.cs

@@ -87,7 +87,7 @@ namespace ICSharpCode.SharpDevelop.Project
 		/// <summary>
 		/// Gets the minimum MSBuild version required to build projects with this target framework.
 		/// </summary>
-		public abstract Version MinimumMSBuildVersion { get; } // TODO: maybe remove this property?
+		public abstract Version MinimumMSBuildVersion { get; }
 		
 		/// <summary>
 		/// Gets whether this is the MS.NET Framework for the desktop. (not Mono, not WinPhone/Win8 app/portable library/...)
@@ -142,6 +142,21 @@ namespace ICSharpCode.SharpDevelop.Project
 			return true;
 		}
 		
+		/// <summary>
+		/// Gets the solution format version corresponding to the older version of Visual Studio that supports this target framework. 
+		/// </summary>
+		/// <remarks>The default implementation of this property is based on the <see cref="MinimumMSBuildVersion"/> property.</remarks>
+		public virtual SolutionFormatVersion MinimumSolutionVersion {
+			get {
+				if (MinimumMSBuildVersion <= Versions.V2_0)
+					return SolutionFormatVersion.VS2005;
+				else if (MinimumMSBuildVersion <= Versions.V3_5)
+					return SolutionFormatVersion.VS2008;
+				else
+					return SolutionFormatVersion.VS2010;
+			}
+		}
+		
 		/* We might implement+use this API in the future if we want to notify the user about missing reference assemblies.
 		/// <summary>
 		/// Tests whether the reference assemblies for this framework are installed on this machine.

src/Main/Base/Project/Src/Project/CompilableProject.cs

@@ -282,6 +282,18 @@ namespace ICSharpCode.SharpDevelop.Project
 			base.Dispose();
 		}
 		
+		public override SolutionFormatVersion MinimumSolutionVersion {
+			get {
+				SolutionFormatVersion v = base.MinimumSolutionVersion;
+				var fx = this.CurrentTargetFramework;
+				// Check if the current target framework has higher requirements than the ToolsVersion:
+				if (fx != null && fx.MinimumSolutionVersion > v) {
+					v = fx.MinimumSolutionVersion;
+				}
+				return v;
+			}
+		}
+		
 		#region IUpgradableProject
 		[Browsable(false)]
 		public virtual bool UpgradeDesired {