[Feature Request]: Set start point of split at ~50%

Thanks for this extension :clap:, I came over to it from WinTile after reading about it in the coverage on OMG Ubuntu's site.

When adding a new split it would be nice for it to start at the 50% point, or there abouts if available space is not divisible by 2. Part of what brought this about was having to use the mouse to try to get an exact value and arrow keys weren't working to move the bar as I expected (and I didn't try it but I didn't see if I could click on the values to set a numerical value)

It's only a minor issue when setting up new layouts in the first instance however this is potentially the first thing that you're trying to do after installing the extension for the first time.