Implement Software Keyboard GTK frontend (#1434)

* Implement SwKbd GUI

* Relocate UI handler to Emu Context from Config

Also create a common interface for UI handlers in the context and specialize for Gtk

Add basic input length validation in InputDialog

* Add Transfer Memory support to AppletCreator

Read Initial Text for SwKbd using Transfer Memory

* Improve InputDialog widget

Improve length validation
Has extra label to show validition info
Handle potential errors and log them

* Misc improvements

* Improve string validation
* Improve error handling
* Remove tuple in struct
* Address formatting nits

* Add proper Cancel functionality

Also handle GUI errors in UI handler

* Address jD's comments

* Fix _uiHandler init

* Address AcK's comments

Ryujinx.HLE/IHostUiHandler.cs

@@ -0,0 +1,14 @@
+using Ryujinx.HLE.HOS.Applets;
+
+namespace Ryujinx.HLE
+{
+    public interface IHostUiHandler
+    {
+        /// <summary>
+        /// Displays an Input Dialog box to the user and blocks until text is entered.
+        /// </summary>
+        /// <param name="userText">Text that the user entered. Set to `null` on internal errors</param>
+        /// <returns>True when OK is pressed, False otherwise. Also returns True on internal errors</returns>
+        bool DisplayInputDialog(SoftwareKeyboardUiArgs args, out string userText);
+    }
+}