[mtouch/mmp] Rework the project files. (dotnet#8575)

* Files outside the project directory now show up with a much more logical
  name in the IDE. External files have a link target which is relative to the
  root xamarin-macios directory, and files included from the mono archive show
  up as such as well.
* Rename a few mtouch/mmp-specific files to contain 'mtouch' or 'mmp' in the
  filename, to avoid having multiple files in the projects with the same name
  (it's confusing every time you search for a filename in the IDE and get
  multiple filenames where only the directory is different).
* Add a tools.sln that contains only the mtouch and mmp projects. This makes
  it easier to work with both mtouch and mmp and the same time, while not
  making VSfM unbearably slow by loading many projects in the same solution.

tools/tools.sln

@@ -0,0 +1,23 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 2012
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "mtouch", "mtouch\mtouch.csproj", "{A737EFCC-4348-4EB1-9C14-4FDC0975388D}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "mmp", "mmp\mmp.csproj", "{F3232882-0FA0-4BB6-9D9C-E2CC779EAF0D}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{A737EFCC-4348-4EB1-9C14-4FDC0975388D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A737EFCC-4348-4EB1-9C14-4FDC0975388D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A737EFCC-4348-4EB1-9C14-4FDC0975388D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A737EFCC-4348-4EB1-9C14-4FDC0975388D}.Release|Any CPU.Build.0 = Release|Any CPU
+		{F3232882-0FA0-4BB6-9D9C-E2CC779EAF0D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{F3232882-0FA0-4BB6-9D9C-E2CC779EAF0D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{F3232882-0FA0-4BB6-9D9C-E2CC779EAF0D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{F3232882-0FA0-4BB6-9D9C-E2CC779EAF0D}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal