UI: Included more launch checks (#4)

- Added Onedrive folder check for Windows.
- Added iCloud and Downloads folder checks for macOS.
- Added sudo checks for macOS/Linux.
- Added dialogue prompts for macOS/Linux.
- Added unofficial build warning for detected Flatpak install.

These checks have console fallbacks in case the GUI decides it doesn't want to work that day.

Reviewed-on: https://git.ryujinx.app/projects/Ryubing/pulls/4

src/Ryujinx/UI/Helpers/LinuxSDLInterop.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.Ava.UI.Helpers
+{
+    public class LinuxSDLInterop
+    {
+        // TODO: add a parameter for prompt style
+        // TODO: look into adding text for the button
+        // TODO: check success of prompt box
+        public static int SimpleMessageBox(string caption, string text)
+        {
+            const string sdl = "SDL2";
+                        
+            [DllImport(sdl)]
+            static extern int SDL_Init(uint flags);
+                        
+            [DllImport(sdl, CallingConvention = CallingConvention.Cdecl)]
+            static extern int SDL_ShowSimpleMessageBox(uint flags, string title, string message, IntPtr window);
+                        
+            [DllImport(sdl)]
+            static extern void SDL_Quit();
+                        
+            SDL_Init(0); 
+            SDL_ShowSimpleMessageBox(32 /* 32 = warning style */, caption, text, IntPtr.Zero);
+            SDL_Quit();
+            return 0;
+        }
+    }
+}