Add configuration option to set a default panel

This allows for customizing the panel that is selected by default when
calcurse is started.

Note that this patch doesn't add any documentation. Also, this
configuration option currently cannot be configured using the
configuration menu.

Implements FR#19.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/vars.c

@@ -134,6 +134,7 @@ void vars_init(void)
   conf.auto_save = 1;
   conf.auto_gc = 0;
   conf.periodic_save = 0;
+  conf.default_panel = CAL;
   conf.system_dialogs = 1;
   conf.progress_bar = 1;
   strncpy(conf.output_datefmt, "%D", 3);