Remove MouseDownThread and its usages

This code comes from an old Be Newsletter and since then the API
received the addition of SetMouseEventMask. In several places the
MouseDownThread was misused: it would spawn a new thread on every mouse
click and not clear the previous one. This could for example lead to
BSpinner skipping values if you clicked it at the right speed.

There are functional changes in BSpinner, before it updated for the
first time 100ms after mouse down, and then as you moved the mouse
around the button, now it activates immediately on first click and then
every 200ms (which may be a bit short). In other places, no functional
changes intended.

Change-Id: Ie600dc68cbb87d1e237633953e5189918bf36575
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2599
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

MouseDownThread: let thread terminate cleanly.

* Killing the thread leaks resources, and it will terminate cleanly and
safely when the destructor exits anyway.
* Fixes #12293. Thanks to ttcoder, jackburton and bonefish for investigating!
* Does not fix #12286. Going to bissect now...

Move Thread classes from Tracker to shared

So that they may be utilized outside of Tracker

MouseDownThread: let thread terminate cleanly.

* Killing the thread leaks resources, and it will terminate cleanly and
safely when the destructor exits anyway.
* Fixes #12293. Thanks to ttcoder, jackburton and bonefish for investigating!
* Does not fix #12286. Going to bissect now...

Move Thread classes from Tracker to shared

So that they may be utilized outside of Tracker