[welcome] Adjust names of URLs to be consistent

- having show*Url and donateUrl seems inconsistent, although
   the show*Url settings were originally boolean-only.
 - add "show" to the Donate button setting, to make them
   all consistent (putting a boolean there will generate a
   warning and hide the button, that's all).

src/modules/welcome/WelcomeViewStep.cpp

@@ -148,7 +148,7 @@ WelcomeViewStep::setConfigurationMap( const QVariantMap& configurationMap )
                            jobOrBrandingSetting( Branding::KnownIssuesUrl, configurationMap, "showKnownIssuesUrl" ) );
     m_widget->setupButton( WelcomePage::Button::ReleaseNotes,
                            jobOrBrandingSetting( Branding::ReleaseNotesUrl, configurationMap, "showReleaseNotesUrl" ) );
-    m_widget->setupButton( WelcomePage::Button::Donate, CalamaresUtils::getString( configurationMap, "donateUrl" ) );
+    m_widget->setupButton( WelcomePage::Button::Donate, CalamaresUtils::getString( configurationMap, "showDonateUrl" ) );
 
     if ( configurationMap.contains( "requirements" )
          && configurationMap.value( "requirements" ).type() == QVariant::Map )

src/modules/welcome/welcome.conf

@@ -19,9 +19,10 @@ showReleaseNotesUrl:    true
 # If this Url is set to something non-empty, a "donate"
 # button is added to the welcome page alongside the
 # others (see settings, above). Clicking the button opens
-# the corresponding link.
+# the corresponding link. (This button has no corresponding
+# branding.desc string)
 #
-# donateUrl:  https://kde.org/community/donations/
+# showDonateUrl:  https://kde.org/community/donations/
 
 # Requirements checking. These are general, generic, things
 # that are checked. They may not match with the actual requirements