Proper support for $(StartProgram)

[KirillOsenkov]

Todd ran into an issue with AddinMaker 1.4.2 on a Mac where having a custom StartProgram defined in his .csproj would result in AddinMaker crashing with Specified cast is not valid:

ERROR [2018-04-16 10:55:47Z]: Execution failed
System.InvalidCastException: Specified cast is not valid.
at MonoDevelop.AddinMaker.AddinProjectFlavor.OnCreateExecutionCommand (MonoDevelop.Projects.ConfigurationSelector configSel, MonoDevelop.Projects.DotNetProjectConfiguration configuration, MonoDevelop.Projects.ProjectRunConfiguration runConfiguration) [0x00009] in <4a319dda1249491a8dda9024f800d5fb>:0
at MonoDevelop.Projects.DotNetProjectExtension.OnCreateExecutionCommand (MonoDevelop.Projects.ConfigurationSelector configSel, MonoDevelop.Projects.DotNetProjectConfiguration configuration, MonoDevelop.Projects.ProjectRunConfiguration runConfiguration) [0x00001] in /Users/todd/src/monodevelop/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProjectExtension.cs:104
at MonoDevelop.Projects.DotNetProject.CreateExecutionCommand (MonoDevelop.Projects.ConfigurationSelector configSel, MonoDevelop.Projects.DotNetProjectConfiguration configuration, MonoDevelop.Projects.ProjectRunConfiguration runConfiguration) [0x00001] in /Users/todd/src/monodevelop/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs:1348
at MonoDevelop.Projects.DotNetProject+d__178.MoveNext () [0x00094] in /Users/todd/src/monodevelop/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs:1847

https://github.com/mhutch/MonoDevelop.AddinMaker/blob/master/MonoDevelop.AddinMaker/AddinProjectFlavor.cs#L78