When upgrading the lexers ensure that any user alterations aren't overridden

The old code failed to call the section that took user-alterations into account, due to m_lexersVersion being set too early in the process.
When fonts and colours have been updated, make sure any session is loaded

Loading a session happens when a wxEVT_COLOURS_AND_FONTS_LOADED event is handled by clMainFrame. This event is normally sent by ColoursAndFontsManager::Load just before the frame is created, so the pending event arrives at the correct time. However the code-path when lexers are updated sends the event earlier, and it before the frame is ready; so send another afterwards.
Use a more informative label when upgrading user themes

The Upgrading banner is shown twice, once for the default lexers, once for the user ones. Change the label the second time through to reflect this.