Theme Editor for Armory2D

This PR adds a theme editor to Armory2D. Video: https://youtu.be/7C55Lykhtco

Features:

Elements follow the theme's colors by default but can overwrite them
Multiple themes can be edited
Theme Editor shows "global" list of themes based on the current project
The unique()-method in Elements.hx is finally implemented
Added an icon to show which theme is the default theme of the canvas @luboslenco Should I commit the .svg file too?

Ideas for the future:

Add support for choosing sizes and all different kinds of theme settings
Rename theme attributes (for example BUTTON_COL doesn't change the button's color only)

armory3d / armory2d