L.Browser.touch should be retired

[johnd0e]

Leaflet/src/core/Browser.js

Lines 98 to 104 in d1a1e97

	// @property touch: Boolean
	// `true` for all browsers supporting [touch events](https://developer.mozilla.org/docs/Web/API/Touch_events).
	// This does not necessarily mean that the browser is running in a computer with
	// a touchscreen, it only means that the browser is capable of understanding
	// touch events.
	export var touch = !window.L_NO_TOUCH && (pointer || 'ontouchstart' in window ||
	(window.DocumentTouch && document instanceof window.DocumentTouch));

At the moment ~95% of browsers support touch events, so this property is not really helpful anymore.
Even more: it is confusing and often mistreated (see #5407, #5266, #3944 and many other).

Currently we have only 9 occurrences in code.

Some are definitely not important

Leaflet/src/dom/Draggable.js

Line 24 in d1a1e97

var START = Browser.touch ? 'touchstart mousedown' : 'mousedown';

Leaflet/src/map/Map.js

Lines 1117 to 1118 in d1a1e97

	DomUtil.addClass(container, 'leaflet-container' +
	(Browser.touch ? ' leaflet-touch' : '') +

Other occurrences should be revised, and most probably we'll find that there will be no harm to consider touch as unconditionally supported.

Control.Layers.js

(this is addressed by #7057)

Leaflet/src/control/Control.Layers.js

Lines 202 to 207 in e213469

	if (Browser.touch) {
	DomEvent.on(link, 'click', DomEvent.stop);
	DomEvent.on(link, 'click', this.expand, this);
	} else {
	DomEvent.on(link, 'focus', this.expand, this);
	}

Tooltip.js

(this is addressed by #7535)

Leaflet/src/layer/Tooltip.js

Lines 109 to 111 in e213469

	if (Browser.touch && !this.options.permanent) {
	events.preclick = this._close;
	}

Leaflet/src/layer/Tooltip.js

Lines 301 to 303 in e213469

	if (Browser.touch) {
	events.click = this._openTooltip;
	}

So I propose to retire L.Browser.touch usage in leaflet codebase.
For compatibility purposes we have to leave L.Browser.touch as true (and describe that in documentation).
More compromise solution is proposed in #7029.

[johnd0e]

Compare: 'touchevents' test is marked deprecated in modernizr.

More info: Modernizr/Modernizr#2432

[johnd0e]

Currently in most places of code touch property check is used to prevent attaching excessive listeners if touch is unsupported.
May be that makes sense for legacy devices (though I doubt that it has any impact on performance with modern ones).