[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?)
bumped version-checks
bumped version-checks for major plist-editing performance fix
[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
Revert "[Core] Cleanup to previous commit"

This reverts commit d68282dd2ea1610f185259354cb3df4d1d546491.
Revert "[Core] Improved perf of Project.Files.GetFile() and GetFileWithVirtualPath()"

This reverts commit 5ac5a74482e98dcb23f65df240e893cd42b4970f.