Updated Swedish locale

It appears that no one is maintaining the Swedish localization of pdf.js any more, since there hasn't been any updates in a couple of months. I'm not the original translator, but I am willing to take over the maintenance of the l10n. (I'm a native Swedish speaker.)

This PR makes a number of changes to the Swedish localization of pdf.js:
* Adds all currently missing fields.
* Fixes the fields who's names have changed since last update.
* A few changes to the `find` strings, so that they correspond to the wording of the Swedish l10n of Firefox.
* A few strings are changed slightly to improve the overall consistence and the grammar of the translation.
* Generally the current translation is good, but are some strings that I find quite confusing. Hence there has been a couple of changes to hopefully improve the overall quality of the translation.