Use stricter compiler settings withing Valama to don't miss warnings.
Refactoring WelcomeScreen: Make it modular.

 * WelcomeScreen is now completely modular.
    - Use a notebook for new project creation steps.
    - Fix some minor issues.
    - Change initial layout a bit (make it more compact).
    - Last page with project name and project location not optimal (doesn't
      look nice). Moved error label in own grid row to not extend the whole
      grid with an invalid character.
    - At the moment it is possible to add new pages with helper class
      TemplatePage or add widgets directly. The latter is more complicated
      because all the low level signal stuff has to be done manually and there
      are also other drawbacks regarding selection state. So most likely the
      widget possibility will be removed later.
 * Save/update recent manager information immediately (after project name
   change). Update list in WelcomeScreen screen.
    - Save information to ~/.cache/valama/recent_projects.xml (add .xml
      extension).
 * Order list of templates by name.
 * Scale template icon to 33x33.
 * Entry: Make default text of error label customizable.
Add valadoc comments to WelcomeScreen.