Add tooltips to buttons

[boatbomber]

Adds tooltips to the buttons. This may seem a bit "extra" for now, but as the Rojo workflow gets more complex (two-way, etc) the single-word buttons aren't enough to convey the meaning behind them, necessitating tooltips. Superfluous tips will likely never been seen by users anyway (since they are unlikely to hover) so there's no detriment to adding them even to simple buttons.

Features:

• Easy usage via Roact component
• Auto size and position using available space
• Tail triangle points at your mouse
• Follows theme and style
• Supports multiple canvases and arbitrary tips

Tech breakdown:
Tooltip.Provider wraps the App, cascading the Context everywhere. Tooltip.Canvas renders any tip popups- it is done this way because Sibling ZIndex means the popups must be in a separate layer above all else. It also allows us to support multiple widgets by drawing Canvases in multiple places. Finally, and this is the part most used, we have Tooltip.Trigger which we put inside any element that we want a popup tip on.

Sample Usage:

Sample = e(TextButton, {
	text = "Sample",
	style = "Bordered",
}, {
	Tip = e(Tooltip.Trigger, {
		text = "This is a sample tooltip"
	}),
})

Demo:

[LPGhatguy]

Nice work! Some review feedback attached.

It's kind of strange that we have to put tooltip triggers inside widgets instead of around widgets. Alas, we live with Roblox's lack of a UI layout model. 😅