DSi Theme: Added more dropdown animations!

[mentusfentus]

What's changed?

• Added more dropdown animations to closely match DSi menu's behaviour. In it, there are 4 dropdown types: Left-to-Right, Right-to-Left, and what I like to call V-Shaped and Inverted V-Shaped (see below). This PR adds these animations for TWLmenu's DSi theme. Also, just like the original, they are randomly picked upon boot.
• While at it, I also cleaned up some of the dropdown code to be more readable and slightly tweaked the old left-to-right animation.

Left-to-Right

ltr.mp4

Right-to-Left

rtl.mp4

V-Shaped

v.mp4

Inverted V-Shaped

iv.mp4

Where have you tested it?

Nintendo DSi XL with Unlaunch
melonDS 0.9.5

Pull Request status

• This PR has been tested using the latest version of devkitARM and libnds.