fix: do not hide contextmenu on movestart

If user opens a contextmenu on touch devices by long tap, in most cases
he also moves the map unintentionally. In most cases this does not fire a
`movestart` event. But if the map has `maxBounds` and the map is bounced
back, a `movestart` event is fired. In this case the contextmenu is hidden
immediately after opening, which totally breaks UX.

Hiding contextmenu on `movestart` event may not be needed at all. As far
as I researched there is no scenario in which the user can move the map
intentionally after contextmenu is opened without firing a `mousedown`
or `pointerdown` event before. Therefor contextmenu is already closed
before `movestart` event is fired.

There is only one scenario I noticed in which a `movestart` is fired
while contextmenu is visible. Using mouse as well as using touch a user
is able to move the map while opening a contextmenu. He could hold
right-click or tab and moving map while opening contextmenu. This
triggers a `movestart` only if he is using a mouse. Since this was
inconsitent between mouse and touch input, I would consider it a bug.

Fixes #94

dist/leaflet.contextmenu.js

@@ -71,7 +71,6 @@ L.Map.ContextMenu = L.Handler.extend({
         this._map.on({
             contextmenu: this._show,
             mousedown: this._hide,
-            movestart: this._hide,
             zoomstart: this._hide
         }, this);
     },
@@ -90,7 +89,6 @@ L.Map.ContextMenu = L.Handler.extend({
         this._map.off({
             contextmenu: this._show,
             mousedown: this._hide,
-            movestart: this._hide,
             zoomstart: this._hide
         }, this);
     },

src/Map.ContextMenu.js

@@ -46,7 +46,6 @@ L.Map.ContextMenu = L.Handler.extend({
         this._map.on({
             contextmenu: this._show,
             mousedown: this._hide,
-            movestart: this._hide,
             zoomstart: this._hide
         }, this);
     },
@@ -65,7 +64,6 @@ L.Map.ContextMenu = L.Handler.extend({
         this._map.off({
             contextmenu: this._show,
             mousedown: this._hide,
-            movestart: this._hide,
             zoomstart: this._hide
         }, this);
     },