Fix F6 shortcut to show the timeline thumbnail by default

[dacap]

There is a "Toggle Timeline Thumbnails" command (F6 key) to toggle the visibility of the timeline thumbnails, but it doesn't work until we change the timeline thumbnail size first, which makes little sense for this command. By default the command should set a size if the current thumbnail size is = 1.

From: https://community.aseprite.org/t/a-way-to-change-the-default-size-of-the-layer-thumbnail/19863/3?u=dacap