Fix for SD-1150

[Expro]

My proposition for fix SD-1150 (http://bugtracker.sharpdevelop.net/issue/ViewIssue.aspx?id=1150&PROJID=4).

Expro

[dgrunwald]

There are some view contents which don't use OpenedFile, but might still belong to the open solution. (for example, "Project Options" and "Project Upgrade")
Others that don't use OpenedFile are indepedent from the solution (Start Page).

So I think we need to add a "bool CloseWithSolution { get; }" to IViewContent. The default implementation (in AbstractViewContent) could then look at the OpenedFiles.

[dgrunwald]

Path applied in 4e54652, and I've added CloseWithSolution in 6ca892e.

[Expro]

Good work. My laptop is broken, my PC mainboard is broken... I am off for few weeks.