actions: only use left mouse button by default

This can be changed for drag and resize actions:

    interact(target)
    .draggable({
      mouseButtons: buttonsMask,
    })
    .resizeable({
      mouseButtons: buttonsMask,
    });

buttonMask may be:

 1 for only the left button (default)
 2 for only the right button
 4 for only the middle (wheel) button

or any logical OR of those values to allow from any one of several
buttons. E.g. 2|4 = 6 for either the right or middle button.

See https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/buttons#Return_value

Close #376

src/actions/drag.js

@@ -7,7 +7,8 @@ const defaultOptions = require('../defaultOptions');
 
 const drag = {
   defaults: {
-    enabled   : false,
+    enabled     : false,
+    mouseButtons: null,
 
     origin    : null,
     snap      : null,

src/actions/resize.js

@@ -11,7 +11,8 @@ const defaultMargin = browser.supportsTouch || browser.supportsPointerEvent? 20:
 
 const resize = {
   defaults: {
-    enabled   : false,
+    enabled     : false,
+    mouseButtons: null,
 
     origin    : null,
     snap      : null,

src/autoStart/index.js

@@ -266,6 +266,13 @@ Interactable.prototype.defaultActionChecker = function (pointer, event, interact
   let action = null;
 
   for (const actionName of actions.names) {
+    // check mouseButton setting if the pointer is down
+    if (interaction.pointerIsDown
+        && interaction.mouse
+        && (event.buttons & this.options[actionName].mouseButtons) === 0) {
+      continue;
+    }
+
     action = actions[actionName].checker(pointer, event, this, element, interaction, rect);
 
     if (action) {

src/defaultOptions.js

@@ -9,6 +9,10 @@ module.exports = {
   perAction: {
     origin: { x: 0, y: 0 },
 
+    // only allow left button by default
+    // see https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/buttons#Return_value
+    mouseButtons: 1,
+
     inertia: {
       enabled          : false,
       resistance       : 10,    // the lambda in exponential decay