Display configurable login message after successful authentication.

Patch by E. Bavier

--- slim-1.3.6/panel.cpp.orig	1969-12-31 18:00:00.000000000 -0600
+++ slim-1.3.6/panel.cpp	2016-10-17 17:00:07.259649063 -0500
@@ -260,6 +260,7 @@
 
 void Panel::ClosePanel() {
 	XUngrabKeyboard(Dpy, CurrentTime);
+	XClearWindow(Dpy, Root);
 	XUnmapWindow(Dpy, Win);
 	XDestroyWindow(Dpy, Win);
 	XFlush(Dpy);
--- slim-1.3.6/cfg.cpp.orig	2016-10-17 17:08:06.767666139 -0500
+++ slim-1.3.6/cfg.cpp	2016-10-17 17:06:53.115663516 -0500
@@ -52,6 +52,7 @@
 	options.insert(option("lockfile","/var/run/slim.lock"));
 	options.insert(option("logfile","/var/log/slim.log"));
 	options.insert(option("authfile","/var/run/slim.auth"));
+	options.insert(option("login_msg","Logging in..."));
 	options.insert(option("shutdown_msg","The system is halting..."));
 	options.insert(option("reboot_msg","The system is rebooting..."));
 	options.insert(option("sessiondir",""));
--- slim-1.3.6/app.cpp.orig	2016-10-17 17:08:41.731667384 -0500
+++ slim-1.3.6/app.cpp	2016-10-17 17:07:48.415665486 -0500
@@ -520,6 +520,8 @@
 	struct passwd *pw;
 	pid_t pid;
 
+	LoginPanel->Message((char*)cfg->getOption("login_msg").c_str());
+
 #ifdef USE_PAM
 	try{
 		pam.open_session();