new setUsernameLineContextualHelp on KPasswordDialog
This adds a new optional UI element next to username field when set. It's effectively a very user visible whats-this button letting the user easily discover that there's more information available.
We need this to tell our users how SMB logon information works. It's the same as Windows' but Linux users tend to not know the various option for Windows logon names. This new feature allows us to not clutter the UI too much but still give the user the ability to easily discover input options that may use.
This is a qwidget-like implementation of a similarly functioning hint idiom found in the QML based fonts KCM.
The help button is not shown unless a help is set. The help is actually set on the user lineedit itself as a whatsthis string and then shown as actual whatsthis window upon button click.
The motivation here is facilitating upstream suggestions for our SMB auth dialog. We'll want to essentially limit the auth dialog to username+password (same as on windows) but there are various input options for the username depending on what environment is in. To explain the options we need a fair amount of space but at the same time it needs to be more easily discoverable than a tooltip. The fonts kcm already solved this in a fairly reasonable way so @ngraham suggested using it here as well. This will also require adjustments in kio and kio-extras but I'd like to get the API for this settled first.
(blurriness due to fractional scaling)