Finetune event triangle position to have the top point at the event time

This should make it easier to then delete events from the context menu.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Manually remove events in the profile context menu

As logical extension of the ability to add bookmarks and gas changes this
adds the ability to remove (any arbitrary) event that happens at the mouse
position (specifically, that is within +/- six (scaled) pixels around the
x-position (time) of the mouse). That's the same width that the triangle
marker occupies which was moved to be centered around the event time in
commit 5752e9742e86 ("Finetune event triangle position to have the top
point at the event time").

Fixes #60

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>