Add a selection widget and non-gui supporting functions for choosing

[David Hampton]

Log Message:
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Add a selection widget and non-gui supporting functions for choosing
accounting periods.

Tags:
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gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/app-utils:
        Makefile.am
    gnucash/src/gnome-utils:
        Makefile.am

Added Files:
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    gnucash/src/app-utils:
        gnc-accounting-period.c
        gnc-accounting-period.h
    gnucash/src/gnome-utils:
        gnc-period-select.c
        gnc-period-select.h