Add `:inhibit-quit` property to improve performance

[fnussbaum]

With this commit one can add the property :inhibit-quit t to a head to prevent the hydra from exiting temporarily before calling that head. (It is intended to be used and only effective on non-exiting heads.)

This greatly improves performance when calling heads repeatedly in quick succession, such as with scrolling or window resizing commands. In such cases the call to hydra-keyboard-quit (and the induced re-creation of the lv buffer and window) can easily become the bottleneck and is often unnecessary. (It is only necessary in special cases, for example, when a head reads from the minibuffer, or when it saves and switches window configurations.)

I would like to apply this option to the scrolling and window resizing hydra heads in Spacemacs, which currently suffer from relatively severe performance problems.