[I18n] Translatable strings

This pull request replaces a lot of **static strings** in the source code with **dynamic "strings.xml"-equivalents**. Since this projects uses _transifex_ the base file should be updated after the merge. Most (all?) of the `values-XX/strings.xml`-files don't contain the xliff document definiton: `xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2"`. This leads to broken translations. The next time any data from transifex is merged, this attribute should be added.