Fix empty help in some Partitioner dialogs (bsc#1194274)

[mvidner]

Problem

Some dialogs in the partitioner have a Help button that pops up an empty window.

• https://bugzilla.suse.com/show_bug.cgi?id=1194274
• https://trello.com/c/22Mos13d

Solution

These fixes work via a method of a base class of all these dialogs, Y2Partitioner::Dialogs::Popup#set_help_text.

A better fix would be to use a common mechanism of CWM+Wizard: Wizard.SetHelpText relying on existence of a widget with Id(:WizardDialog) but that is risky for SLE15-SP4

Testing

Tested manually. For easier testing, the dialogs are:

• MkfsOptions "Format Options" Add Partition, Next, Next, Options, Help
• FstabOptions "Fstab Options" Add Partition, Next, Next, Mount Device, Fstab Options, Help
• ImportMountPoints "Import Mount Points from Existing System" System menu, Import Mount Points (in installation mode)
• PartitionTableClone "Clone partition layout of /dev/sdx" Device menu, Clone Partitions to Another Device, Help

Screenshots

Working help popups with this fix:

[yast-bot]

:heavy_check_mark: Internal Jenkins job #5 successfully finished :heavy_check_mark: Created IBS submit request #270066