Use g_spawn_async() on Windows

Remove Windows-specific code to spawn processes. Bundle the GLib spawn
helper in the Windows installer.
Refactor process spawning code

Refactor the process spawning code so that it handles additional
arguments, as it did before switching to use g_spawn_async(). Use a
single et_run_program() method for spawning processes, and merge the
existing implementations.
Add folder icons and emblem to NSIS installer
Default to an icons style toolbar on Windows
Launch application after NSIS installation
Use LOCALAPPDATA for NSIS user data uninstall