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This contains a major rework of DataSource support in Ozone Wayland
backend. Just like other recent refactors in data exchange related code,
such as https://crrev.com/c/2205881, the primary goal of this one is to
decouple "client code" from core implementation and interface of
*DataSource classes, so making it easier to maintain, extend, read, etc
etc as well as maximizing code reusage and will reduce complexity to
implement use cases such as Tab/Window Dragging.

To accomplish it, this CL:

1. Switches DataSource implementations to a template-based (instead of
using inheritance) approach in order to support *slightly* different
backing protocol extensions and maximize code reusage and consistency
on what is shared. For now there are 2 variants, one for the standard
wl_data_source protocol object and another one for gtk-primary-selection
extension (and probably a new one coming soon [2]).

2. Introduces DataSource::Delegate interface, through which the relevant
data source events are forwarded to "client code", eg: WaylandClipboard,
WaylandDataDragController, etc.

3. Refactors DataSource clients, mainly WaylandClipboard, making use of
the new template class instances to reach a cleaner and easier to extend
implementation of PlatformClipboard, seamlessly supporting both regular
and primary selection clipboard buffers.

4. Get rid of unneeded classes and source files.

No functional changes is expected and follow up changes should be done
in the future to switch the Device and Offer counterparts to a similar
architecture.

[1] https://gitlab.gnome.org/GNOME/mutter/-/blob/master/src/wayland/protocol/gtk-primary-selection.xml
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=1088132

Bug: 896640
Change-Id: Iaf035dd106c6ab9488f44cfd8fb425ff3cd354db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222147Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#776903}