Make the startup application dialog look like a normal dialog

If it quacks like a dialog, then it should also look like a dialog,
which means no headerbar (regular titlebar instead), and a Close
button at the bottom.

https://bugzilla.gnome.org/show_bug.cgi?id=707837
Updated Spanish translation
Updated Slovenian translation
Updated Polish translation
Updated Galician translations
Updated Lithuanian translation
Updated Brazilian Portuguese translation
startup: add the "+" button as a Tweak

This makes sure that there is always at least one tweak in the
group, which is necessary for the tweak model to show it.

https://bugzilla.gnome.org/show_bug.cgi?id=707840
Updated Spanish translation
Updated Czech translation
Updated Czech translation
Updated Brazilian Portuguese translation
Updated Polish translation
Fix a translatable string

So the old gettext would pick it up. Not a string freeze break.
l10n: Update Japanese translation
Updated Hebrew translation
Updated Hebrew translation
Updated Danish translation
Make Header Bar match mockups
Tidy and dist appdata file