Frontend.XF.GTK: exit with one when user is root

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

```
update.go:85: cannot change mount namespace according to change mount (/snap/gtk-common-themes/1534/share/gtk2/Materia-compact /snap/geewallet/x1/data-dir/themes/Materia-compact none bind,ro 0 0): cannot write to "/snap/gtk-common-themes/1534/share/gtk2/Materia-compact" because it would affect the host in "/snap"
update.go:85: cannot change mount namespace according to change mount (/snap/gtk-common-themes/1534/share/gtk2/Materia-dark-compact /snap/geewallet/x1/data-dir/themes/Materia-dark-compact none bind,ro 0 0): cannot write to "/snap/gtk-common-themes/1534/share/gtk2/Materia-dark-compact" because it would affect the host in "/snap"
update.go:85: cannot change mount namespace according to change mount (/snap/gtk-common-themes/1534/share/gtk2/Yaru-MATE-dark /snap/geewallet/x1/data-dir/themes/Yaru-MATE-dark none bind,ro 0 0): cannot use "/snap/gtk-common-themes/1534/share/gtk2/Yaru-MATE-dark" as bind-mount source: not a directory
update.go:85: cannot change mount namespace according to change mount (/snap/gtk-common-themes/1534/share/gtk2/Yaru-MATE-light /snap/geewallet/x1/data-dir/themes/Yaru-MATE-light none bind,ro 0 0): cannot use "/snap/gtk-common-themes/1534/share/gtk2/Yaru-MATE-light" as bind-mount source: not a directory
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

@@ -37,6 +37,10 @@ module Main =
     [<EntryPoint>]
     [<STAThread>]
     let main argv =
+        if GWallet.Backend.Config.IsLinuxPlatform() && Environment.UserName = "root" then
+            Console.Error.WriteLine "Running as root user is not supported"
+            Environment.Exit 1
+
         match argv.Length with
         | 0 ->
             NormalStartWithNoParameters()