Move clients/ to plugins/kdecorations/

Summary:
KWin's plugins are now all in a plugins subdirectory. This is a good
argument to also move the window decoration plugins there. The name
clients was not really good anyway and makes it difficult for people
not familiar with the code base to find it. Having it under plugins
is the more expectable location.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1414

CMakeLists.txt

@@ -322,9 +322,6 @@ if(KWIN_BUILD_KCMS)
     add_subdirectory( kcmkwin )
 endif()
 
-if( KWIN_BUILD_DECORATIONS )
-    add_subdirectory( clients )
-endif()
 add_subdirectory( data )
 
 add_subdirectory( effects )

plugins/CMakeLists.txt

@@ -2,3 +2,7 @@ add_subdirectory(kglobalaccel)
 add_subdirectory(qpa)
 add_subdirectory(idletime)
 add_subdirectory(platforms)
+
+if(KWIN_BUILD_DECORATIONS)
+    add_subdirectory(kdecorations)
+endif()