[[Feature]] Add OS and Platforms to LCB Metadata

Update *Extending LiveCode* guide with note and example about adding
metadata for OS and/or Platforms if the module uses code that is only
available on specific machines.

Updated `__fetchMetadatum` to include the new keys in the manifest.xml
file.

Author: bwmilby

Documentation/guides/Extending LiveCode.md

@@ -107,9 +107,16 @@ in the IDE and in the online portal.
 	metadata title is "My Pink Circle"
 	metadata author is "Benjamin Beaumont"
 	metadata version is "1.0.0"
+	metadata platforms is "desktop,mobile"
 
 	end widget
 
+> **Note:** If the module makes use of external code that is only
+> available on specific Operating Systems or Platforms, use the "os"
+> and/or "platforms" metadata keys. The values are the same as can be
+> found in the LiveCode Documentation Format Reference. This data will
+> also appear in the dictionary.
+
 #### Importing libraries
 The LiveCode builder syntax is broken down into **modules**. There are 3 
 classes of module:
@@ -830,7 +837,7 @@ pseudo-randomly generated UUID as a string:
 
 	metadata title is "Java UUID"
 	metadata author is "LiveCode"
-	metadata version is "1.0.0"	
+	metadata version is "1.0.0"
 
 	// Bind to the static randomUUID() method of the java.util.UUID class
 	__safe foreign handler JNI_RandomUUID() returns JObject \

Toolset/libraries/revidedeveloperextensionlibrary.livecodescript

@@ -568,7 +568,7 @@ end revIDEDeveloperExtensionNew
 private function __fetchMetadatum pXmlId, pKey
    local tNode
 
-   if pKey is among the items of "name,type,title,author,description,version" then
+   if pKey is among the items of "name,type,title,author,description,version,platforms,os" then
       -- Some metadata are placed into specifically-named top-level XML
       -- elements in the manifest.  This list must be kept up-to-date
       -- with the list of special-cased metadatum keys listed in the