Change previously highlighted text's highlight color

[mmounirf]

Describe the bug
When you highlight text it's being stored in highlighted items, if you selected the same context and change the highlight color, it's being saved again, same value with two colors

To Reproduce
Steps to reproduce the behavior:

1. Select text to highlight
2. Click on marker color
3. Check that it is now existing in stored highlights
4. Select the same text you selected in step 1
5. Click on marker color (choose different color)
6. Text is being saved twice with different values

Expected behavior
The value stored should be replaced with new color

Screenshots
N/A

Desktop (please complete the following information):

• OS: Ubuntu 18.04.1 LTS
• Browser Version 69.0.3497.100 (Official Build) (64-bit)
• Version 0.0.4

Additional context
N/A