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Mouse handling
Arcballs pretend there is an invisible sphere around an object and when you
drag, the point on the sphere that was under the mouse at the beginning should
remain under the mouse at the end.
There are some good resources on the web if you search for "virtual trackball"
or something like that.
You can apply the same principle moving the camera left/right/up/down: pretend
there's a plane in front of the camera (perpendicular to the view ray), find the
point that is initially under the mouse then translate to ensure that the point
is still under the mouse when you release.
This will mean unprojecting the screen coordinates from the previous and current
mouse positions and subtracting the previous from the current to find the
translation vector. Add that vector to the camera position & that should be it,
I think?
A decent (and mercifully brief!) explanation of quaternions is at: