Add KeePassXC

[romain-dartigues]

Thank you for your contribution! Before sending this Pull Request, please confirm the following:

• I have read the contributing guidelines
• I have run python check.py and received a ✅
• The icon's size is under 1,024 Bytes
• The layout of the SVG looks like this including newlines

<svg xmlns="http://www.w3.org/2000/svg"
aria-label="..." role="img"
viewBox="0 0 512 512"><path
d="m0 0H512V512H0"
fill="#fff"/> ... </svg>

If you have done the above, please send the Pull Request.

Questions:
I did took some liberty to overly simplify the colours, but as there are no strict official guidelines, I ask your opinion if you prefer this version or the monochrome one