[client] ui: add input:helpMenuDelay option

This option controls the time period (in ms) after which the help menu
appears when holding down the escape key. After this time period,
capture mode is no longer toggled.

This fixes #527.

client/src/main.h

@@ -137,6 +137,7 @@ struct AppParams
   bool              captureOnStart;
   bool              quickSplash;
   bool              alwaysShowCursor;
+  uint64_t          helpMenuDelayUs;
 
   unsigned int      cursorPollInterval;
   unsigned int      framePollInterval;