Add touch input support

This patch builds upon the prior work by @4z3 and @bytbox to add
touchscreen support to Alacritty. While some inspiration was taken from
@4z3's patch, it was rewritten from scratch.

This patch supports 4 basic touch interactions:
 - Tap
 - Scroll
 - Select
 - Zoom

Tap allows emulating the mouse to enter a single LMB click. While it
would be possible to add more complicated mouse emulation including
support for RMB and others, it's likely more confusing than anything
else and could conflict with other more useful touch actions.

Scroll and Select are started by horizontal or vertical dragging. While
selection isn't particularly accurate with a fat finger, it works
reasonably well and the separation from selection through horizontal and
vertical start feels pretty natural.

Since horizontal drag is reserved for selection we do not support
horizontal scrolling inside the terminal. While it would be possible to
somewhat support it by starting a selection with vertical movement and
then scrolling horizontally afterwards, it would likely just confuse
people so it was left out.

Zoom is pretty simple in just changing the font size when a two-finger
pinch gesture is used. Performance of this is pretty terrible especially
on low-end hardware since this obviously isn't a cheap operation, but it
seems like a worthwhile addition since small touchscreen devices are
most likely to need frequent font size adjustment to make output
readable.

Closes #3671.

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