fix(dropdown) Changed "slide down" transition to "slide"

[krpatel5657]

Description

Previously, dropdown menus would close behind other form elements if the two were vertically located next to each other. The fix changes the default transition for dropdown menus when opening or closing to be the "slide" method, instead of "slide down." This results in the dropdown menu closing on top of other form elements. Also, there was an error with dropdown menus that open upwards, as shown in the gifs below. These menus would then slide even further up to close, instead of collapsing down on the form field. The change to the "slide" transition also fixed this issue, again as shown by the gifs.

Testcase

https://jsfiddle.net/krpatel5657/hqe1pnu5/4/

Screenshot (if possible)

Original issue:

Fix:

Simultaneous fix with upwards dropdown menus:
Original error:

Fix:

Closes

#2663

[ko2in]

Your PR introduces the new issue. When you open the dropdown, it overlaps by the next following menu item until it's fully opened.

I don't think this issue relates to dropdown module alone, but transition module also.

[lubber-de]

Your change does only "heal the wounds", but the real reason for the issue is a double hide transition which messes up both (underlaying menu and upward hiding)
I fixed that by #2818 and will close this PR