Respect touchscreen to output mapping from wlroots

wlroots' touchscreen objects (struct wlr_touch) have the "output_name" field, which specifies the corresponding output device, from wlroots' own point of view. Use these mappings if present not explicitly overridden by gsettings.

This fixes the issue where touch inputs do not get rotated with the screen when changing device orientation on setups where wlroots provides this information (such as when running nested).

Helps: #312