Manually added grades are removed when toggling the grades of existing subjects

[runarmod]

This is a quick solution to another problem when the manual addition of grades was implemented. At first, the manually added grades would stay in the table, but their grades would not be reflected in the grade average.

Possible solutions:

• after searching for grades that are checked, search through the table of manually added grades and take them into account
• ... or start with the manually added grades instead of resetting to 0 when toggling a checkbox
• ... or do not recount every time a checkbox is toggled, but rather only update the one grade toggled when checkbox is pressed (I think this is the best way to do it)