Implements the approach suggested by jralls in PR #2222: instead of a
context menu action, introduce GncCellRendererLabel - a
GtkCellRendererText subclass that returns a read-only, selectable
GtkEntry when a cell is activated.
All numeric columns (Balance, Total, Cleared, Reconciled, Present,
Future Minimum, and their report/period variants) are affected because
they all go through gnc_tree_view_add_numeric_column.
Usage:
- Click once to select the row (unchanged behaviour)
- Click again on a numeric cell to activate it; the value appears in a
read-only entry field with the text pre-selected
- Ctrl+C copies the value to the clipboard; Escape or clicking away
dismisses the field without modifying the model
- Unicode bidi control characters (added by GnuCash for RTL display)
are stripped before showing the value so the copied text is clean