…Disposable)

+ Also adds an regular MSBuild Properties and other related changes for convenient work with this properties like extension method GetFileProperties, AddOrUpdate etc.

Part of 3F/vsSolutionBuildEvent#53
WIP

…sSolutionBuildEvent:

3F/vsSolutionBuildEvent#53
#17

## IConfPlatform changes:

+`bool IsEqualByRule(string config, string platform, bool icase = false);`

    Checking an config/platform by using {Rule} instance.

## IXProjectEnv changes:

+`IXProject XProjectByFile(...);`

    Find project by full path to file.

+`IEnumerable<IXProject> Assign(IEnumerable<Project> projects = null);`

    Assign an existing `Microsoft.Build.Evaluation.Project` instances for local collection.

+`IXProject AddOrGet(Project project);`

    Adds `Microsoft.Build.Evaluation.Project` instance into IXProject collection if it does not exist.

+`ProjectItemCfg ExtractItemCfg(Project project);`

    Prepares data from `Microsoft.Build.Evaluation.Project` instance.

+`void UnloadAll(bool throwIfErr = true);`

    Unloads all evaluated projects at current time.

+`bool Unload(IXProject xp);`

    Unloads specified project.

+`IEnumerable<Project> ValidProjects`

    List of valid projects such as something except `.user` but contains FirstChild / LastChild XML node.

## ISlnResult changes:

+`string SolutionFile`

    Full path to an solution file.

## New extension methods:

```
+TVal GetOrDefault<TKey, TVal>(this IDictionary<TKey, TVal> data, TKey key, TVal def)
+bool IsEqual(this Project a, Project b)
+string GetSolutionDir(this Project eProject)
+string GetProjectRelativePath(this Project eProject)
+string GetConfig(this Project eProject)
+string GetPlatform(this Project eProject)
+string GetSolutionExt(this Project eProject)
```

## Other

* Removed pGuid checking from `IXProjectEnv.GetOrLoadProject()`
* Added `FileExt` type for work with `ProjectType` via its file only.

also added method extensions:

* `bool Remove<T>(this IList<T> src, T elem, Func<T, T, bool> comparer)`
* `bool IsLimEqual(this IXProject x, IXProject prj)`

and added .metaproj filter for an `IXProjectEnv.ValidProjects` property