[Debugger] Fixed heuristics for hoisted "this" references for async

Fixes bug #9345.
[Debugger] Nullables should use DebuggerDisplayAttribute of the type

Fixes bug #9046 (and I guess bug #6282?)
[Debugger] Added special support for displaying Nullable<T>'s

Fixes bug #6282
[Debugger] Allow copying multiple items in ObjectValueTreeView

Now behaves closer to VS (except nicer clipboard formatting).

Fixes bug #3195
[Debugger] Optimized previous patch for SDB (i.e. avoid extra round-tripping)
[Debugger] Make System.Nullable check more robust
[Debugger] Allow multiple breakpoints per line

Fixes bug #2851
[Core] Improved perf of Project.Files.GetFile() and GetFileWithVirtualPath()

This actually improves loading of the MonoDevelop project for me by ~1 second.

Also really improves performance of the plist editor for a big customer's app.
[Core] Cleanup to previous commit