Custom gesture detector

[renancaraujo]

As discussed on @criswonder's #185, The proper way to deal with nested gesture detectors is implementing our own Gesture recognizer. In this PR:

• We add a corner-hit mechanic;
• Taking advantage of this mechanic, the new PhotoViewGestureRecognizer determines if it wins or not the GestureArena based on the position of the child in relation to the corners;
• Renamed PhotoViewImageWrapper to PhotoViewCore;
• Made a reorganization on src directories;
• Removed usePageViewWrapper option from the gallery;
• Removed unnecessary function typedefs like PhotoViewScaleStateChangedCallback;
• Now the corner hitting behavior on the gallery (Regarding Let user scroll to next page when PageView child is zooming (#41) #185 and Can gallery mode support 'detect image edge' but not unlock when contained? #41 ) works on vertical galleries.
• Added vertical gallery example.

[ghost]

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[criswonder]

Bro, nice job!
I tried the master branch's code, but it not work as expected! It's hard to change page when zoomed in. like this gif shows pls have a look

[renancaraujo]

@criswonder Yeah... That looks awful, on iOS and android devices that I've tested, I had not much trouble with this impl, but for sure we still have to look into some cases. Can you open an issue telling Flutter version, device model and android version?