Correct a typo: "choosen" -> "chosen"

The typo is in the src/build_tool_dialog.vala, but also appears in the
English part of the translation files, so this patch modifies them
without making them fuzzy (this is a funny case: the original text is
wrong, but the translations are supposed to be correct!).
Add keywords to latexila.desktop

It is now recommended for desktop programs to include keywords in their
desktop entry. These keywords can then be used by application launchers.