gitg-repository-list-box: avoid getting row from event y

fix: #110

No idea why, but the event's y location is per row rather from the listbox top. This has the effect of, when right clicking the second row at its top-most position, it will be 0, instead of first_rows_y + event.y. Then, when you select the row at y 0, it will be the first one.

This MR follows a different approach. Rows are responsible for keeping track of their secondary clicks and emit the appropriate signal. The listbox connects to that signal and marks the row that emitted it as selected (if the listbox wasnt in selection mode)