Add the active keyboard modifiers to the mouse events

[robbert-vdh]

This builds on top of #115, so I marked this as a draft until that has been merged.

Every mouse event now has a modifier field containing the current keyboard modifiers. Most window management APIs work like this, and it solves things like #116 by making sure that whenever a window receives a mouse event, it also knows the up to date keyboard modifier status.

I tested this on Linux with the project mentioned in #116. It should also work without issues on Windows though, and I've never touched macOS code before but it compiles and looks fine so I'm sure it also works there.

[robbert-vdh]

I've rebased this on the master branch now that #115 has been merged.