Update to 2.10.6

Changelog:
Overview of Changes from GIMP 2.10.4 to GIMP 2.10.6
===================================================

Core:

  - Render drawable previews asynchronously.
  - Merge the file view filter and file format lists in GimpFileDialog.
    The presence of 2 lists was very confusing.
  - DLL search priority is now updated before running a plug-in on
    Windows, depending on the executable bitness. This gets rid of one
    of the last remnant of DLL hell in GIMP, which was when running
    32-bit plug-ins from a 64-bit build of GIMP.

Filters:

  - New "Little Planet" (gegl:stereographic-projection) filter.
  - New "Long Shadow" (gegl:long-shadow) filter.

Tools:

  - Halt the Measure tool after straightening.
  - Add an "orientation" option to the measure tool, corresponding to
    the "orientation" property of GimpToolCompass (i.e., it controls the
    orientation against which the angle is measured, when not in 3-
    point mode.)  The orientation is "auto" by default, so that the
    angle is always <= 45 deg.  Note that the "orientation" option
    affects the tool's "straighten" function, so that the layer is
    rotated toward the current orientation.
  - Text layers can now represent vertical texts, with 4 variants:
    left-to-right and right-to-left lines, and forcing all characters to
    be upright or following Unicode's vertical orientation property.
    See also:
    * https://www.unicode.org/reports/tr50/
    * http://www.unicode.org/Public/UCD/latest/ucd/VerticalOrientation.txt

User Interface:

  - The Dashboard dockable dialog now has an "async" field to the
    dashboard's "misc" group, showing the number of async operations
    currently in the "running" state.
  - New Preferences option to enable/disable layer-group previews, since
    these can get quite time-expensive.

Translations:

  - New language: Marathi
  - 12 translations were updated: Brazilian Portuguese, Dutch, French,
    German, Greek, Italian, Latvian, Polish, Romanian, Slovenian,
    Spanish, Swedish.

Build:

  - Add --with-win32-32bit-dll-folder configuration option to override
    the folder where 32-bit versions of DLL will be installed (default:
    32/bin/).
  - Install all plug-ins in their own directories. Unlike on master,
    this is not mandatory to do so, but it would protect our core
    plug-ins against any DLL installed directly under plug-ins/ by
    third-party plug-ins (cf. Windows DLL hell).


(ryoon)