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/calcurse.h

@@ -233,6 +233,15 @@
 #define MAX(x,y) ((x)>(y)?(x):(y))
 #define MIN(x,y) ((x)<(y)?(x):(y))
 
+enum win {
+  CAL,
+  APP,
+  TOD,
+  NOT,
+  STA,
+  NBWINS
+};
+
 /* General configuration variables. */
 struct conf {
   unsigned auto_save;
@@ -240,6 +249,7 @@ struct conf {
   unsigned periodic_save;
   unsigned confirm_quit;
   unsigned confirm_delete;
+  enum win default_panel;
   unsigned system_dialogs;
   unsigned progress_bar;
   const char *editor;
@@ -449,15 +459,6 @@ struct binding {
   enum key action;
 };
 
-enum win {
-  CAL,
-  APP,
-  TOD,
-  NOT,
-  STA,
-  NBWINS
-};
-
 #define FLAG_CAL (1 << CAL)
 #define FLAG_APP (1 << APP)
 #define FLAG_TOD (1 << TOD)