LanguageClient: Add "Clear" button to LSP inspector

It's helpful to be able to watch new messages from a certain point
onwards, and not be distracted by older ones.

Change-Id: I0506dad24d4b970652117d7e704ebfb4b49cad74
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/lspinspector.h

@@ -85,6 +85,7 @@ public:
     std::list<LspLogMessage> messages(const QString &clientName) const;
     Capabilities capabilities(const QString &clientName) const;
     QList<QString> clients() const;
+    void clear() { m_logs.clear(); }
 
 signals:
     void newMessage(const QString &clientName, const LspLogMessage &message);