Added keyboard movement

[Toniman575]

Simply added constant keyboard movement to the do_camera_movement system and MoveMode and DirectionKeys fields to PanCam and registered them in Bevy.

As I do not know if this change is even desired, this seemed to be the least intrusive API/code change.
Further changes I thought about but discarded because of reason above:

• Option for accelerated movement.
• Splitting the do_camera_movement into multiple systems.
• Simultaneous mouse and keyboard MoveMode.

Addresses #52

[johanhelsing]

I think this makes sense scope-wise. Curious to hear anybody else's opinion as well...

API-wise: I think simultaneous movement should be implemented and be the default. Perhaps even the only mode, and remove the mode field. If you don't want keyboard controls, you can set direction_keys = DirectionKeys::NONE.

Curious for other thoughts on this as well.

[Toniman575]

The implementation I went for seemed the most sane to me but I'm curious what u guys think.
Further thoughts:

• We could move projection.area.size() behind if checks so it doesn't happen if unnecessary, but the performance gain seems rather insignificant and I am hesitant to add code complexity.
• I think check_egui_wants_focus makes enough sense as it is right now, but we could check for wants_pointer_input and wants_keyboard_input separately depending on the input of the user. The easiest way seems to be to split do_camera_movement system into two separate systems for keyboard movement and mouse movement. Again though, this might be a bit too intrusive of a change, especially as this change would only be relevant for a crate feature.

[johanhelsing]

Thanks, API-wise, I think this adds it in a way that fits well with the scope and the rest of this crate.

We should update the readme/crate docs, maybe examples and this is good to go :)

[Toniman575]

I added a bit of Documentation, but I'm not sure if its too wordy so I'll defer to @johanhelsing on that. Otherwise I'm fairly happy with the changes, hope u guys are as well.

[johanhelsing]

Great work!

Thanks also @tomara-x for reviewing!

❤️