While a glyph/action is executing, queue next glyph/action

A simple method to queue a single, subsequent glyph/action in the hotbar while the character is performing a glyph/action would go a long ways toward making the combat feel smoother and the character feel more responsive to the user's input.

As an example, suppose the user presses hotkey 1 to cast spell A. Before the global hotbar cooldown elapses, the user can press hotkey 2 so that spell B begins casting immediately after the global hotbar cooldown elapses.

Some scenarios to further explain:

1) In the above example, if the user pressed hotkey 2 and then hotkey 3 and then hotkey 4 while the global cooldown is still active (i.e. while spell A is still casting), then only hotkey 4 would be queued. In other words, the last hotkey pressed (and only the last hotkey pressed) during the global cooldown will be performed after the global cooldown expires.

2) Only execute the queued hotkey if it is available to be executed at the time that the global cooldown elapses. For example, if spell B had a glyph specific cooldown (e.g. you had cast it 10 seconds prior, and it has a 30 second recast delay), then the queued hotkey would not execute when the global hotbar cooldown elapsed.