LibGUI: Add GUI::Process::spawn_or_show_error()

This is a wrapper around Core::Process::spawn() for GUI applications,
that shows an error if the call to spawn() failed.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -79,6 +79,7 @@ set(SOURCES
     PasswordInputDialog.cpp
     PasswordInputDialogGML.h
     PersistentModelIndex.cpp
+    Process.cpp
     ProcessChooser.cpp
     Progressbar.cpp
     RadioButton.cpp