Added summary plots

Added summary plots:

- percentage indicators for subjects
- bar plots of session hours in the last 7 days

First prototype

For now, only adding and editing subjects is supported.

What's Changed

• Improved the computation of weekly study by @ZinRicky in #1

New Contributors

• @ZinRicky made their first contribution in #1

Full Changelog: https://github.com/ZinRicky/StudyTime/commits/v0.1.1