Command margin font

This is a cosmetic change to use the same font face and font size for the command margin as is used in the main text editor.  It improves ease of use when typing commands and searching and makes the command margin more vim-like.  Hard-coding the command margin font was trivial; doing it right involved a fair amount of work.

The main idea is that we use the "Fonts and Colors" properties to retrieve the current text editor font face and font size and then update it when the user applies configuration changes.  The implementation encapsulates the needed information into a new test-friendly `IFontProperties` interface.  I updated all the tests that use the command margin and all related tests still pass.  For reference, there does not appear any way to do this (get font information) by using `EditorFormatDefintion` and friends.

I believe the new code adheres to the coding style of the VsVim code base but I might have overlooked something.  Some new assembly references were needed but I think that they are all safe and nothing new is referenced that wasn't already referenced by at least one other common assembly.