Frontend.XF.GTK: different exitCode on root

An error occurs when geewallet is run with root permissions,
thus it’s necessary to restrict users from running it as root:

```
mkdir: cannot create directory ‘/run/user/0’: Permission denied
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
realpath: '': No such file or directory
Authorization required, but no authorization protocol specified

(geewallet:5797): Gtk-WARNING **: 09:01:13.760: cannot open display: :0
```

src/GWallet.Backend/Config.fs

@@ -47,6 +47,9 @@ module Config =
     let IsMacPlatform() =
         RuntimeInformation.IsOSPlatform OSPlatform.OSX
 
+    let IsLinuxPlatform() =
+        RuntimeInformation.IsOSPlatform OSPlatform.Linux
+
     let GetMonoVersion(): Option<Version> =
         FSharpUtil.option {
             // this gives None on MS.NET (e.g. UWP/WPF)

src/GWallet.Frontend.XF.Gtk/Program.fs

@@ -39,6 +39,10 @@ module Main =
     let main argv =
         match argv.Length with
         | 0 ->
+            if GWallet.Backend.Config.IsLinuxPlatform() && Environment.UserName = "root" then
+                Console.Error.WriteLine "Running the GTK GUI as root is not supported"
+                Environment.Exit 1
+
             NormalStartWithNoParameters()
         | 1 when argv.[0] = "--version" ->
             Console.WriteLine (SPrintF1 "geewallet v%s" VersionHelper.CURRENT_VERSION)