WIP: file-operations.c: Button style in dialog

Ghost User requested to merge (removed):issue-969 into master

Currently, in simple dialog's buttons, "destructive-action" style is only applied to buttons with "DELETE" string.

However, there are buttons that have destructive actions but with different string, like "Empty Trash".

This commit add the possibility to change the style for each button, and possibly to add new styles.

Close #969 (closed)

Edited by António Fernandes

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