GestureDetector: Make two_finger pans & swipes report the same sort of data than their standard brethren

[NiLuJe]

In the process of looking over the "swipe reports the contact point (i.e., gesture start) as pos while pretty much everything else reports the lift point (i.e., gesture end)" for the QuickMenu anchoring thingy, I realized that the two-finger variants of the pans & swipes were doing things... differently (by always using the "swipe" semantics, which effectively means that two_finger_pan reports completely different things than pan).

So, I started looking into best to handle that, and I came up with an ugly mess.

Which leads me to a simple question: why? Wouldn't it be much, much simpler is swipes just behaved like everything else and reported the lift point as pos?

I don't think we actually care about it much... unless it's used for the tap/gesture zone hit detection, in which case it makes sense to use the start position, dammit (yes, I just thought about that right now :D).

Anyway, RFC, I haven't even started testing this anyway ;D.

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This change is 

[NiLuJe]

Hmm, tweaking the hit-detection is probably as simple a tweaking GestureRange's match function, but then I still need some nastiness to make sure only actual swipes have a start_pos set...

Sooo, it's probably still cleaner to leave the nastiness inside GestureDetector, and let everyhting else be happy in the fact that pos will always be what the gesture was triggered for, oblivious to the fact that it may not be the lift point...

[NiLuJe]

Didn't fit in the title, but this also switches the QuickMenu anchor the the lift point for swipes, re: #10636 ;).

[NiLuJe]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @NiLuJe)

[hius07]

Can you please keep the dispatcher unchanged for a while, to avoid conflicts with #10646.

[NiLuJe]

Oops, thought it was in an unmodified chunk ;).

[NiLuJe]

Reviewed 1 of 1 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @NiLuJe)

[hius07]

Dispatcher is ready to get gesture end_pos when it appears.