A button that revert the current .blend

Question

How can I create a button that calls "bpy.ops.wm.revert_mainfile()" when pressed?

Answer 1

Just pass the identifier of the operator to UILayout.operator(operator_identifier). Example based on the Panel Simple template that comes with Blender (adds a panel to the Object Properties):

import bpy
class HelloWorldPanel(bpy.types.Panel):
    """Creates a Panel in the Object properties window"""
    bl_label = "Hello World Panel"
    bl_idname = "OBJECT_PT_hello"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "object"
def draw(self, context):
    layout = self.layout

    row = layout.row()            
    row.operator("wm.revert_mainfile")



def register():
    bpy.utils.register_class(HelloWorldPanel)
def unregister():
    bpy.utils.unregister_class(HelloWorldPanel)
if name == "main":
    register()

Further reading: How to create a custom UI?