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/mainwindow.cpp

@@ -57,6 +57,7 @@
 #include "progressview.h"
 #include "shortcutsettings.h"
 #include "vcsmanager.h"
+#include "variablechooser.h"
 
 #include "scriptmanager_p.h"
 #include "settingsdialog.h"