Allow setting horizontal and vertical alignment separately (#2072)

* Add ability to declare horizontal and vertical alignment separately

* Add docs

examples/user_guide/Customization.ipynb

@@ -170,6 +170,45 @@
     "    pn.Column(pn.widgets.Button(name='Run', margin=(25, 50, 75, 100)), background='#f0f0f0'))"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## ``align``\n",
+    "\n",
+    "The `align` parameter controls how components align vertically and horizontally. It supports 'start', 'center', and 'end' values and can be set for both horizontal and vertical directions at once or for each separately by passing in a tuple of the form `(horizontal, vertical)`.\n",
+    "\n",
+    "One common use-case where alignment is important is when placing multiple items with different heights in a `Row`:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "pn.Row(pn.widgets.IntSlider(name='Test'), pn.widgets.IntSlider(align='end'))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "In a grid you may also have to specify the horizontal and vertical alignment separately to achieve the layout you are after:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "pn.GridBox(\n",
+    "    pn.widgets.IntSlider(name='Test'), pn.widgets.IntSlider(align='end'),\n",
+    "    pn.widgets.TextInput(name='Test', width=150), pn.widgets.TextInput(width=150, align=('start', 'end')),\n",
+    "    ncols=2)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

panel/viewable.py

@@ -41,10 +41,11 @@ class Layoutable(param.Parameterized):
     for all Panel components with a visual representation.
     """
 
-    align = param.ObjectSelector(default='start',
-                                 objects=['start', 'end', 'center'], doc="""
+    align = param.ClassSelector(default='start',
+                                class_=(str, tuple), doc="""
         Whether the object should be aligned with the start, end or
-        center of its container""")
+        center of its container. If set as a tuple it will declare
+        (vertical, horizontal) alignment.""")
 
     aspect_ratio = param.Parameter(default=None, doc="""
         Describes the proportional relationship between component's