LibGUI: Add GInputBox for getting a string from a modal dialog.

Use this to implement some of the toolbar actions in IRCClient. :^)

LibGUI/GInputBox.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <LibGUI/GDialog.h>
+
+class GInputBox : public GDialog {
+public:
+    enum ExecResult { ExecOK = 0, ExecCancel = 1 };
+
+    explicit GInputBox(const String& prompt, const String& title, GObject* parent = nullptr);
+    virtual ~GInputBox() override;
+
+    String text_value() const { return m_text_value; }
+
+private:
+    void build();
+    String m_prompt;
+    String m_text_value;
+};