VariableChooser: Rework

Allow multiple expanders to be registered for lineedits, e.g. a
local and the global ones, and actually show them.

Use a tree view in the chooser for somewhat more structured display.

Change-Id: I769f92144e5249f45e54381de52aa6973eb20118
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/coreplugin/externaltool.cpp

@@ -33,7 +33,6 @@
 #include "actionmanager/actionmanager.h"
 #include "actionmanager/actioncontainer.h"
 #include "coreconstants.h"
-#include "variablemanager.h"
 
 #include <app/app_version.h>
 #include <coreplugin/icore.h>
@@ -41,8 +40,10 @@
 #include <coreplugin/documentmanager.h>
 #include <coreplugin/editormanager/editormanager.h>
 #include <coreplugin/editormanager/ieditor.h>
-#include <utils/qtcassert.h>
+
 #include <utils/fileutils.h>
+#include <utils/macroexpander.h>
+#include <utils/qtcassert.h>
 #include <utils/qtcprocess.h>
 
 #include <QCoreApplication>
@@ -54,6 +55,7 @@
 
 #include <QDebug>
 
+using namespace Utils;
 using namespace Core;
 using namespace Core::Internal;