Add touchpad drag-lock related settings

[hanaral]

Problem

The current touchpad mechanism to drag seems like a monolithic, rather inaccessible solution that is only there because it was there before. I know that people who have less dexterous fingers (whether that is a medical condition or age, it shouldn't matter) struggle to double-tap-to-drag.

Proposal

Add options to change the mode of touchpad drag locking such as:

• Double tap to lock the drag until clicked again
• 3 Finger drag (disables ability to use 3 finger workspaces)

This is the way macOS has handled it for a while and it works really well (I much prefer 3 finger dragging as opposed to double tapping)

Prior Art

[JoseExposito]

For the record, this option is available in libinput:

Also optional is a feature called “drag lock”. With drag lock disabled, lifting the finger will stop any drag process. When enabled, libinput will ignore a finger up event during a drag process, provided the finger is set down again within a implementation-specific timeout. Drag lock can be enabled and disabled with libinput_device_config_tap_set_drag_lock_enabled(). Note that drag lock only applies if tap-and-drag is be enabled. https://wayland.freedesktop.org/libinput/doc/latest/tapping.html#tapndrag

I'm not sure if we could easily handle this "accessibility" feature, but I'd be nice to have.

[hanaral]

Doesn't gala have to communicate with libinput to change sensitivity anyway?