installer: Update strings and change concept

The following changes were made:

• Strings now comply with the HIG (title case, no periods in single sentence descriptions, no unnecessary ellipses)
• Buttons better communicate what they do (Not Now -> Test)
• Complicated wording has been simplified (evaluated -> tested)
• Search boxes use the common convention (Search for languages… -> Search languages)
• The warning page has a clearer title, but I'm still not that sure about it (Disclaimer Title -> Pre-Release)
• Buttons are less verbose (Begin Installation -> Install…)
• I removed the row in the "No existing data" scenario, as it doesn't need one. The description already communicates that the entire disk is used, and there is nothing to wipe regardless.