refactor: change how slides are displayed

[immadisairaj]

Before we duplicate the presentationwidget background for every slide when where is no slide background. Now, this changes by having a constant background widget and the previous slides will be transparent.

Closes #5

Description

Slide Stack in PresentationWidget:

• before - [ Slide 1 | Slide 2 | Slide 3 ] (background is appended based on slide background
• after - [ background | Slide 1 (transparent) | Slide 2 (transparent) | Slide 3 ]

Checks

• [x] Updated/Added the relevant tests and checked them before PR
• [x] Updated/Added all the documentation changes
• [ ] Updated/Added the relevant examples in the /examples folder

Breaking Change

This doesn't change much (can't be considered as breaking change). But, has uses:

1. Reduces re-creating of background widgets
2. Enables background customisation
3. Reduces problems of having previous slide displayed behind the current slide

[immadisairaj]

A case - where one wants to animate a slide on entry, the previous slide has to be displayed.

Need to think on how to solve this.

[immadisairaj]

We can probably provide an option to display previous slide behind the current slide. Maybe an option set by the user. Opening a new issue on this.