Make width and height snap to fit text

Currently we can resize the app however we like. This gives fairly ugly margins on the bottom and right hand side of the window where there is not enough space to fit another character. These margins are especially noticeable in applications that draw their own background (eg. (Neo)Vim) where the background of the terminal is visible in these margins.

It would be better to make the window snap to sizes that fit an exact amount of characters.