Mac: Add automatic code signing and notarization

[cwensley]

This adds the ability to create DMG's, code sign, and notarize your Eto.Mac64 apps automatically. This only works on a Mac.

New properties that control these features:

DMG Creation

    <!-- add to your csproj -->
    <EnableDmgBuild>True</EnableDmgBuild>

    <!-- File Name of the DMG -->
    <DmgName Condition="$(DmgName) == ''">$(MacBundleName)</DmgName>
    <!-- DMG volume name -->
    <DmgVolumeName Condition="$(DmgVolumeName) == ''">$(DmgName)</DmgVolumeName>
    <!-- Where to put the DMG -->
    <DmgOutputPath Condition="$(DmgOutputPath) == ''">$(OutputPath)</DmgOutputPath>

    <!-- True to add typical formatting to the DMG, add application shortcut icon, background, auto-open, etc. -->
    <DmgBeautify Condition="$(DmgBeautify) == ''">True</DmgBeautify>

    <!-- Size of the icons in the DMG -->
    <DmgIconSize Condition="$(DmgIconSize) == ''">72</DmgIconSize>
    <!-- Bounds of the Finder window when opened (left, top, right, bottom) -->
    <DmgBounds Condition="$(DmgBounds) == ''">400, 100, 900, 430</DmgBounds>
    <!-- Location of your app icon (x, y) -->
    <DmgAppLocation Condition="$(DmgAppLocation) == ''">120, 150</DmgAppLocation>
    <!-- Location of the applications shortcut icon (x, y) -->
    <DmgApplicationsShortcutLocation Condition="$(DmgApplicationsShortcutLocation) == ''">380, 150</DmgApplicationsShortcutLocation>
    <!-- Image to use for the background, or 'none' to omit the background image. -->
    <DmgBackgroundImage Condition="$(DmgBackgroundImage) == ''">$(MSBuildThisFileDirectory)Dmg.background.png</DmgBackgroundImage>

Code Signing:

    <!-- add to your csproj -->
    <EnableCodeSigning>True</EnableCodeSigning>

    <!-- Search key for the code signing identity for the .app (partial strings match)
         If empty, this will use the code signing identity if only a single identity is installed.
     -->
    <CodeSigningKey Condition="$(CodeSigningKey) == ''"></CodeSigningKey>
    <!-- Search key for the code signing identity for the .dmg (partial strings match) -->
    <DmgCodeSigningKey Condition="$(DmgCodeSigningKey) == ''">$(CodeSigningKey)</DmgCodeSigningKey>
    <!-- Code signing entitlements to use -->
    <CodeSignEntitlements Condition="$(CodeSignEntitlements) == ''">$(MSBuildThisFileDirectory)CodeSign.entitlements</CodeSignEntitlements>

Notarization:

    <!-- add to your csproj -->
    <EnableNotarization>True</EnableNotarization>

    <!--
      Wait for notarization to complete and staple the DMG.
      Note that this can make the build take a long time waiting for your app to be notarized.
      -->
    <EnableNotarizationStapler Condition="$(EnableNotarizationStapler) == ''">True</EnableNotarizationStapler>

    <!-- Identifier for the notarization, which is returned in the notarization email to identify what has been notarized -->
    <NotarizationBundleIdentifier Condition="$(NotarizationBundleIdentifier) == ''">$(DmgName).dmg</NotarizationBundleIdentifier>

    <!-- The notarization user name. The username can be inferred from the password keychain entry if not specified. -->
    <NotarizationUserName Condition="$(NotarizationUserName) == ''"></NotarizationUserName>

    <!--
      The notarization password. To use from the keychain (the default), use the following command:

      xcrun altool - -store-password-in-keychain-item "AC_PASSWORD" -u "<NotarizationUserName>" -p <secret_password>
    -->
    <NotarizationPassword Condition="$(NotarizationPassword) == ''">@keychain:AC_PASSWORD</NotarizationPassword>

Fixes #1748