Add WordClouds gramplet

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Summary

Adds three word-cloud gramplets — Given Name Word Cloud, Surname Word
Cloud, and Place Word Cloud — that render names/places as a
size-weighted, click-to-navigate word cloud instead of the old plain-text
link list.

• wordcloudwidget.py — a Cairo/Pango WordCloudWidget (Gtk.DrawingArea)
  that lays out words with a spiral placement algorithm, sized and colored
  by frequency.
• slideroption.py — a SliderOption/GuiSliderOption pair (a
  NumberOption rendered as a slider + text entry), registered via
  BasePluginManager.register_option() so no changes to Gramps core are
  required.
• cloudgramplet.py — shared CloudGramplet base class handling the
  common option set (top-N count, colors, layout quality, filter
  missing/unknown values) and word-cloud wiring.
• givennamewordcloudgramplet.py, surnamewordcloudgramplet.py,
  placewordcloudgramplet.py — the three concrete gramplets.

This is based on ideas from ClmntPnd's Gramps core PR
gramps-project/gramps#2223,
which added the WordCloudWidget-based rendering (including a Place Cloud
gramplet and autosave_options support) directly to Gramps core. This PR
adapts that work into a self-contained addon instead, so it:

• Works with unmodified, current stable Gramps releases (no core patches).
• Uses new gramplet ids/class names (Given Name Word Cloud /
  GivenNameWordCloudGramplet, Surname Word Cloud /
  SurnameWordCloudGramplet, Place Word Cloud /
  PlaceWordCloudGramplet) that are distinct from the built-in
  Given Name Cloud / Surname Cloud gramplets, to avoid any plugin
  registration collision between the addon and the core gramplets of the
  same name.
• Uses the standard Save-button option flow (rather than the core-patched
  autosave_options instant-apply behavior), since that required a
  grampletpane.py change.

Test plan

• tests/test_imports.py — all modules import cleanly; each gramplet
  class is a proper Gramplet subclass.
• tests/test_wordcloudwidget.py — font-size/color interpolation and
  bounding-box overlap helpers.
• tests/test_slideroption.py — SliderOption value/min/max/step
  behavior and GuiSliderOption widget wiring (run under xvfb-run,
  since it needs a real display connection).
• tests/test_cloudgramplet_logic.py — get_items() for each
  gramplet against a real in-memory Gramps database, including the
  filter_missing option and backlink-based place filtering.
• Manual: install the addon, open each of the three gramplets in a
  running Gramps 6.1, verify word clicks navigate correctly and
  options apply on Save.