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/UWP.Images/UWP.Images.projitems

@@ -29,6 +29,7 @@
     <Content Include="$(MSBuildThisFileDirectory)globe.png" />
     <Content Include="$(MSBuildThisFileDirectory)icon.png" />
     <Content Include="$(MSBuildThisFileDirectory)id.png" />
+    <Content Include="$(MSBuildThisFileDirectory)ion_chevron_left.png" />
     <Content Include="$(MSBuildThisFileDirectory)ion_chevron_right.png" />
     <Content Include="$(MSBuildThisFileDirectory)launch.png" />
     <Content Include="$(MSBuildThisFileDirectory)lightbulb.png" />