Vector icons with night-mode support

[kvn1351]

Description

I've redesigned the icons while trying to keep some form of familiarity with the original ones as not to alienate current users.

Advantages:

• They're vector
• They're still editable with Illustrator or similar software
• They work great in night mode
• They reflect this addon better in my humble opinion

Please replace the space inside the brackets with an x and fill out the ellipses if the following items apply:

• I've read and understood the contribution guidelines
• I've tested my changes against at least one of the following Anki builds:
  • Latest standard Anki 2.1 binary build
• I've tested my changes on at least one of the following platforms:
  • Linux, version:
  • Windows, version:
  • macOS, version: