Be able to resize the splits to the same size.

Here how it looks like: https://imgur.com/a/uaMQvT6

Before, it was impossible to resize the splits
with the same size in case they were manually resized.

I have added a new button menu for doing so:
in View -> Split View -> Equal size to all views.

First I wanted to make this as a plugin,
but since I thought it could make splits a bit more useful,
I decided to merge it not as a plugin but internally.

GUI: Menu -> View -> Split View -> Equal size to all views
CHANGELOG: Added equal size to all split views.