Show URL placeholder hint in wizard

Show URL placeholder hint in wizard

In owncloudsetupnocredspage.ui, the URL input field leUrl has a
placeholder text saying "https://..." which is a very useful hint
for the user.  However, in the OwncloudSetupPage constructor, the
placeholer text is overwritten by the return string of the theme's
wizardUrlHint() method.  The NextcloudTheme class does not override
this virtual method, so an empty string is used.

To make available the "https://..." hint, it is moved from the
UI file to NextcloudTheme::wizardUrlHint().  Note that, if a
theme is used which does not allow a custom server URL, the
placeholder text is now empty.  This makes sense because the
input field is disabled in that case.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>