added SHIFT_SPACE_IS_TAB as compile time parameter

for the mobile layout, we want shift+space to produce tab , which needed
to be implemented in the main code. For the desktop layout, we don't want
this though, a config parameter now handles this at compile time.

Ref: https://github.com/jjsullivan5196/wvkbd/pull/103

config.mobintl.h

@@ -3,6 +3,7 @@
 
 #define DEFAULT_FONT "Sans 14"
 #define DEFAULT_ROUNDING 5
+#define SHIFT_SPACE_IS_TAB
 static const int transparency = 255;
 
 struct clr_scheme schemes[] = {