Add a Keywords entry to the .desktop file + minor typo fix

The Keywords entry is described in the [desktop file specification](http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html). Adding it is recommended by the [Debian](http://lintian.debian.org/tags/desktop-entry-lacks-keywords-entry.html) and [Ubuntu](https://wiki.gnome.org/GnomeGoals/DesktopFileKeywords) packaging policies.

Replaced 'specifed' with 'specified' in `PAppearanceWidget.ui` and all affected .ts files.