Add a generic variable chooser widget.

The chooser allows choosing variables from a list, and inserting them
into previously focused line edit, text edit, or plain text edit.
Variables are now registered with a description in the variable manager.
Also make the QT_INSTALL_BINS --> CurrentProject:QT_INSTALL_BINS for
consistency.

src/plugins/coreplugin/variablemanager.h

@@ -67,6 +67,11 @@ public:
     QString value(const QString &variable, const QString &defaultValue);
     Utils::AbstractMacroExpander *macroExpander();
 
+    void registerVariable(const QString &variable,
+                          const QString &description);
+    QList<QString> variables() const;
+    QString variableDescription(const QString &variable) const;
+
 signals:
     void variableUpdateRequested(const QString &variable);