Overview of changes in gcalctool 5.21.2

* Fixed bug #485919 - Use Glade for UI. Glade conversion of gcalctool 
  is now complete (thanks Robert).

* Fixed bug #495349 - Close precision dialog when setting precision from
  Acc button.

* Fixed bug #495345 - Connect "activate" signal for precision spin button
  after spin button has been updated.

* Fixed bug #482655 - Missing image file.

* Fix the buglet in comment #51 of bug #485919

* Fixed for final remaining parts of bug #488694 - [a11y] Several 
  calculator buttons don't speak correctly with Orca.

* Fixed bug #488489 - Make accuracy shortcuts only work in scientific mode.

* Fixed bug # 488703 - Stop bogus '-/-' appearing in accuracy menu items.

* Fix for bug #485134 - OR calculation produces incorrect result in
  arithmetic precedence mode.

* General code cleanup.

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