[Splash screen]: Remove float on top property.

Fixes #868. Splash screen now starts on top of all windows, but other windows can be raised above it. 

UPDATE: @guilhermito reports that on Windows 32, Windows 64, Ubuntu, and Kubuntu clicking on the splash screen makes it disappear. However, both @peabody124 and I have noticed that this behavior does not work on OSX. In addition, I have tested this on Linux Mint and seen the same behavior as OSX. It seems that the "click to destroy" behavior is system dependent.