Debugger: Reduce tooltip flicker.

Move tooltip-request handling from DebuggerPlugin into
DebuggerToolTipManager.

Request tooltip only if position changed and close
standard tooltip on success.

src/plugins/debugger/watchutils.h

@@ -60,6 +60,7 @@ namespace Internal {
 class WatchData;
 class GdbMi;
 
+bool isEditorDebuggable(Core::IEditor *editor);
 QByteArray dotEscape(QByteArray str);
 QString currentTime();
 bool isSkippableFunction(const QString &funcName, const QString &fileName);