Refactor the "proxy action" behavior of Command to Utils::ProxyAction

A generic action that acts as a proxy for another (changeable) action,
like it is used for mostly all of the visible actions in Qt Creator
through the action manager.

src/libs/utils/utils.pro

@@ -7,3 +7,9 @@ CONFIG += dll
 include($$PWD/../../qtcreatorlibrary.pri)
 
 include(utils-lib.pri)
+
+HEADERS += \
+    proxyaction.h
+
+SOURCES += \
+    proxyaction.cpp