Introduce a custom scale gesture detector implementation

[LukasPaczos]

Closes #39.

This PR introduces a custom scale gesture detector, not relying on the one provided in the support library.
Additionally:

• Fixes an issue where a minimal required span for multi-pointer gesture has not been set.
• Correctly interrupts quick scale gesture when another pointer is placed.
• Removes the test introduced in Guard against move events coming from different view trees #71 as it's no more applicable with a changed motion event consumption. The example now consistently reproduces the issues, but the stack trace comes from the ScrollView, which is expected.
• Sets the minimal required span for any multi-pointer gesture to 6mm for API >= 24 or 10mm otherwise. This has been used only for the scale gesture before but now is replacing 55dp for all gestures (which wasn't set anyway because of the first bullet point).
• Introduces a quick scale as another progressive gesture that can be filtered separately from the traditional pinch-to-zoom gesture (AndroidGesturesManager#GESTURE_TYPE_QUICK_SCALE).

[tobrun]

looks and feels great!