Reverse the gradient in battery segment

Closes #910
Document how to create gradient colors
Use battery_full and battery_empty for displaying hearts

Red hearts for battery full are still more optimal in case white hearts are for
empty (though I do not understand why they have to be red). Cannot agree about
per cents though: red is better for empty then white is if there are no related
parts to compare.
Add two more colors to white_red gradient cterm colors

Picked up with

    tools/generate_gradients.py 231 223 216 209 196
Regenerate gui colors for white_red gradient

Command used to generate colors:

    tools/generate_gradients.py -C 231 255 223 216 209 202 196 --weights='1 1 1 1 1 1 3'
Merge pull request #914 from ZyX-I/better-battery-colors

Improve battery segment colors