Decompile 0x006B4800

My first pull request where I decompiled something, so please be critical!

This is a start for decompiling the construction window.

Apparently to create the construction window, the ride window first has to be created, and then by manually calling the mouse event handler of the ride window the construction window is created.

Pretty hacky, haha.

Relative discussion:
https://github.com/IntelOrca/OpenRCT2/commit/78cd7407faea0daa23487f0656ac8e63335a18aa#diff-c2151f8a716a67551868b3f297435a05R448