LibGUI: Allow more programmatic control over GUI::InputBox

This will be needed for WebDriver, which will require constructing and
controlling dialogs manually. Currently, InputBox will only set its text
value when the OK button is pressed. This changes InputBox to update its
text when done(ExecResult::OK) is invoked in any way.

This also makes the text_value() method public, allows for setting the
text value, and allows for moving-in the initial text value.

Userland/Libraries/LibGUI/Dialog.cpp

@@ -115,6 +115,8 @@ void Dialog::done(ExecResult result)
     if (!m_event_loop)
         return;
     m_result = result;
+    on_done(m_result);
+
     dbgln("{}: Quit event loop with result {}", *this, to_underlying(result));
     m_event_loop->quit(to_underlying(result));
 }