[QGIS-Developer] Dropping "spatial query" plugin

[Nyall Dawson]

Hi all,

I've just opened https://github.com/qgis/QGIS/pull/5170, which drops
the old core "Spatial Query" plugin.

Because the functionality of this plugin is now better handled by the
Processing c++ "Extract by Location" and "Select by Location"
algorithms, in my opinion this plugin can be safely dropped now.

The processing algorithms have the benefits of:

- a familiar, more modern interface
- ability to be included in multi-step models
- ability to be used in batch modes
- always available in all QGIS installs (as opposed to the always
installed, but disabled-by-default spatial query plugin)
- unit tests
- non-blocking execution
- more optimised code
- features can be extracted to a new layer regardless of input layer
type (spatial query only allows this for spatialite, postgres and some
other formats)

There's one feature which is available in the spatial query plugin
which isn't present in the processing algs - the interactive list of
affected feature ids (clicking features in the list will zoom to
them). However - I don't consider this list alone to be worth keeping
the plugin alive for, since most users won't find a list of raw
feature IDs particularly helpful.

Please speak up if you think there's still a requirement to keep this
plugin around, otherwise it will be dropped for 3.0!

Nyall