C++: use a global string table for SearchSymbols.

This string table uniques strings, so that multiple identical strings
share their contents. It is used by the locator and the symbol searcher,
and will later be used by the class view.

Change-Id: Ib8b50f69bbf994d0d7a39b66dc8caf1a3d9bfb42
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cpptools.pro

@@ -52,6 +52,7 @@ HEADERS += \
     includeutils.h \
     insertionpointlocator.h \
     searchsymbols.h \
+    stringtable.h \
     symbolfinder.h \
     symbolsfindfilter.h \
     typehierarchybuilder.h
@@ -104,6 +105,7 @@ SOURCES += \
     includeutils.cpp \
     insertionpointlocator.cpp \
     searchsymbols.cpp \
+    stringtable.cpp \
     symbolfinder.cpp \
     symbolsfindfilter.cpp \
     typehierarchybuilder.cpp