Fix immediate popup on start

[rene-descartes2021]

Changes:

• Fixed problem in which_key#start(). If description dictionary is small, then even if user types fast the window will show immediately, as getchar(1) is reached quickly and is false, then window shows.
  • A problem with this fix is if the dictionary is large, then the cumulative delay could be around 2 seconds... Could be mitigated with use of reltime(), if has('reltime').
  • Related: g:which_key_timeout doesn't seem to work #158. Perhaps there could be a boolean setting added here which allows the end-user to show the popup menu immediately without delay, if they wanted.

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Stuff that will be in other PRs:

• [WIP] Fix regression in :WhichKey! {dict} form? Cache shouldn't be shared between the two forms?