release 0.15.11

[forrestguice]

• adds "create/restore backup" option; saves all configuration data as json text (closes [Feature request] Backup Option for App-Settings ... #757).
• adds "export/import widget" option; save/load individual widget configurations.
• adds support for restoring all widgets from backup (requires launcher implementing AppWidgetManager.ACTION_APPWIDGET_RESTORED).
• adds donation link to the about dialog; adds Liberapay to donation options (closes Add Liberapay to the donate options #574); https://liberapay.com/forrestguice/
• fixes bug in date widget where the scaled text is too large (closes Very big font size for date widget #763).
• increments CalculatorProviderContract version 6 -> 7; adds columns for event position data.
• updates translation to Polish and Esperanto (eo, pl) (translations to Polish and Esperanto #761 by Verdulo). translation to pl (Polish) #662 translation to eo (Esperanto) #663
• updates translations to Simplified Chinese (zh_CN) and Traditional Chinese (zh_TW) (feat: updated Chinese translations #765 by James Liu). translation to zh_TW (Traditional Chinese) #680 translation to zh-CN (Simplified Chinese) #681