Cleanup Confirm Drawing Delete dialog

[Phergus]

Describe the bug
Cleanup of code, adding comments & javadocs, use appropriate JOptionPane members, add notes to Crowdin concerning the I18N strings for same.

Note: The I18N strings used in the dialog have mnemonic keys defined but because of the custom text used for the buttons the JOptionPane dialog won't use them which is why the & is stripped out before use.

[Phergus]

Set appropriate dialog type and set default (Enter key) to be No.
Replaced magic numbers with proper JOptionPane values.
Replaced dump of stacktrace in getBackupManager() to showError() call because it was right there.
Updated/added javadocs.

[Phergus]

No visible to changes for end users unless an error starting the backup manager should occur.