Improve the navigation on Windows platforms (#199)

* App: Don't default to modal pushes on Windows

On Windows we generally want to avoid modal pushes as that doesn't give
us a back arrow on Windows 10. Default to non-modal pushes.

Signed-off-by: Alistair Francis <alistair@alistair23.me>

* App: LoginPage.cs: Don't add the cancel button on Windows

With Windows Phone having a HW back buton and the UWP toolbar now
supporting a back button we no longer need a cancel button.

Signed-off-by: Alistair Francis <alistair@alistair23.me>

* src: Set a icon for UWP cancel buttons

We can't always use modal pages and in that case we need to set a icon
for UWP otherwise the button can't be seen on the toolbar.

Signed-off-by: Alistair Francis <alistair@alistair23.me>

* App: Pages: Don't add Cancel/Close button on Windows

Signed-off-by: Alistair Francis <alistair@alistair23.me>

* Update DismissModalToolBarItem.cs

src/App/Controls/DismissModalToolBarItem.cs

@@ -1,4 +1,5 @@
 using Bit.App.Resources;
+using Bit.App.Utilities;
 using System;
 using Xamarin.Forms;
 
@@ -15,13 +16,14 @@ namespace Bit.App.Controls
             // TODO: init and dispose events from pages
             InitEvents();
             Text = text ?? AppResources.Close;
+            Icon = Helpers.ToolbarImage("ion_chevron_left.png");
             Priority = -1;
         }
 
         protected async override void ClickedItem(object sender, EventArgs e)
         {
             base.ClickedItem(sender, e);
-            await _page.Navigation.PopModalAsync();
+            await _page.Navigation.PopForDeviceAsync();
         }
     }
 }