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/coreplugin.pro

@@ -90,7 +90,8 @@ SOURCES += mainwindow.cpp \
     rssfetcher.cpp \
     externaltool.cpp \
     dialogs/externaltoolconfig.cpp \
-    toolsettings.cpp
+    toolsettings.cpp \
+    variablechooser.cpp
 
 HEADERS += mainwindow.h \
     editmode.h \
@@ -178,7 +179,8 @@ HEADERS += mainwindow.h \
     rssfetcher.h \
     externaltool.h \
     dialogs/externaltoolconfig.h \
-    toolsettings.h
+    toolsettings.h \
+    variablechooser.h
 
 FORMS += dialogs/newdialog.ui \
     actionmanager/commandmappings.ui \
@@ -186,7 +188,8 @@ FORMS += dialogs/newdialog.ui \
     dialogs/openwithdialog.ui \
     editormanager/openeditorsview.ui \
     generalsettings.ui \
-    dialogs/externaltoolconfig.ui
+    dialogs/externaltoolconfig.ui \
+    variablechooser.ui
 RESOURCES += core.qrc \
     fancyactionbar.qrc