Check page_theme.php and improve theme name/description handling

What about checking `page_theme.php` before installing new themes?
What about adding `getThemeName` and `getThemeDescription` to the class defined in `page_theme.php`?
This pull request tries to solve both.

BTW it's **still a work in progress**: I don't know which is the fully qualified class name implemented by themes inside packages... For stand-alone themes it's `\Application\Theme\TheHandleOfMyTheme\PageTheme`, but for packages? Is it something like `\Application\Package\TheHandleOfMyPackage\Themes\TheHandleOfMyTheme\PageTheme`?