Doc: don't use the term SDK in connection with Qt

Replace with "Qt distributions" or "installers" or rewrite
in some other way.

Remove references to installing Qt 4 SDK, because it is
no longer available for downloading.

Change-Id: I291f82702e21d953f2efbb0e634d6454b4bc17b9
Reviewed-by: hjk <hjk121@nokiamail.com>

doc/src/widgets/qtdesigner-plugins.qdoc

@@ -47,8 +47,8 @@
 
     \QD fetches plugins from the standard locations and loads the plugins
     that match its build key. \QD is delivered both as a standalone application
-    and as part of the SDK, where it is integrated into \QC. The correct folder
-    to place the plugins depends on which one you use.
+    and integrated into \QC. The correct folder to place the plugins depends on
+    whether you use the standalone \QD or the integrated \QD.
 
     The integrated \QD fetches plugins from the \c {%SDK%\bin\designer} folder
     on Windows and Linux. For information about how to configure plugins on
@@ -128,7 +128,7 @@
 
     \section1 Matching Build Keys
 
-    The \QC that is included in pre-built SDK packages on Windows is built with
+    The \QC that is included in pre-built Qt packages on Windows is built with
     the Microsoft Visual Studio compiler, whereas the version of Qt shipped for
     building applications is configured and built to use the MinGW/g++ compiler.
     Plugins built by using this version of Qt cannot be loaded by \QC because