UI: Make font size to UI elements size ratio consistent

Size of most of the GUI elements is given as a fixed percentage of the screen size (or a child node). [Font size](https://docs.rs/bevy/0.9.1/bevy/text/struct.TextStyle.html) (e.g. of button caption) is influenced by other properties (probably DPI?). 

Therefore the physical text size to the GUI element sizes varies across devices. Such behavior is not desired, the ratio should stay constant.