Create Preference Window

[karbassi]

Now that there are a few other simple features/settings requested, we should have a preferences window option.

[robinstewart]

@karbassi Do you mean an app-wide preferences window or a per-timer settings panel?

So far the feature requests I've looked at seem per-timer, like setting a name, a custom sound, a custom volume.

[robinstewart]

Also it occurs to me that some of this we can do via regular menu items (instead of or in addition to a panel):

[karbassi]

@robinstewart if it's per-timer, then maybe a slide-out panel from the timer with a few settings.

I think we should have global hotkey preference panel if we need it.

For example, this is Spectacle's.

[robinstewart]

Sounds good.

[karbassi]

We could use https://github.com/sindresorhus/Preferences for this too.