glib: remove unused 'using' statements
glib: ObjectManager class can be made static
MSBuild: change some projects to Library type, not Executable

Even though gtk-sharp cannot be built via MSBuild (yet?), it's better
to make the .csproj reflect more closely what you get with the normal
build.
MSBuild: remove AssemblyInfo.cs where it's not present

The only autogenerated AssemblyInfo files are in cairo and in
gtk-sharp's root, not inside other libraries like atk, glib, etc.
Removing them will make MonoDevelop stop rendering a red element
underneath each project.
glib: mark MSBuild project with AllowUnsafe setting

In the same way it is needed for other projects such as gtk.csproj.
glib: reference .NET 3.5's System.Core in MSBuild project

glib.csproj needs to reference System.Core from .NET 3.5 because
glib-sharp now[1] uses System.Action [2] delegate (not to confuse
with System.Action<T>) which was not available in .NET 2.0's System
reference.

[1] https://github.com/mono/gtk-sharp/commit/8e07e7d2257b185866dedfda9cbf32b2e00cd3ee
[2] http://msdn.microsoft.com/en-us/library/system.action%28v=vs.90%29.aspx