[client] input: add support for evdev keyboard capture mode

The new configuration option `input:evdev` accepts a comma separated
list of `/dev/input/` keyboard devices to use for input when in capture
mode. This makes it possible to capture only a specific keyboard instead
of all keyboards.

client/include/interface/displayserver.h

@@ -183,6 +183,9 @@ struct LG_DisplayServerOps
   void (*capturePointer)(void);
   void (*uncapturePointer)(void);
 
+  /* get the character code for the provided scancode */
+  int (*getCharCode)(int sc);
+
   /* exiting = true if the warp is to leave the window */
   void (*warpPointer)(int x, int y, bool exiting);
 
@@ -253,6 +256,7 @@ struct LG_DisplayServerOps
   DEBUG_ASSERT((x)->ungrabPointer      ); \
   DEBUG_ASSERT((x)->capturePointer     ); \
   DEBUG_ASSERT((x)->uncapturePointer   ); \
+  DEBUG_ASSERT((x)->getCharCode        ); \
   DEBUG_ASSERT((x)->warpPointer        ); \
   DEBUG_ASSERT((x)->realignPointer     ); \
   DEBUG_ASSERT((x)->isValidPointerPos  ); \