alt-backtick on desktop icon whilst alt-tabbing prevents further alt-tabbing

# Alt-Backtick on Desktop Icon Whilst Alt-Tabbing Prevents Further Alt-Tabbing

## Version information

  * `uname -r` -> `4.4.0-22-generic`
  * `unity --version` -> `unity 7.4.0`

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## Steps

  * `alt-tab` until desktop icon is highlighted
  * `alt-backtick` on desktop icon
  * Attempt to `alt-tab` once more

## What happens

Further input such as `alt-tab` and `alt-backtick` is ignored until I stop pressing them.

## What I think this should do

I believe it should allow me to continue to `alt-tab` to another icon. This would not lead to any bad behaviour.

## Why I think this is a bug

When you select an icon other than the desktop icon, it is possible to continue `alt-tab`. With a small number of tabs this can be annoying if you accidentally start to `alt-backtick` in the desktop icon instead of the one you wanted to.