Grammar & style of i18n labels

Mostly very minor grammar and/or consistency fixes in
translatable labels, full sentences in help labels, etc.
Two label asignments are replaced with GTK+ stock items which
results in correct translations, standard mnemonics, optional
button icons, ... These two are `GO_FORWARD` and `APPLY`. The
latter replaces "Install" on the installation overview screen.
Merge pull request #34 from kernc/labels_fixes

Grammar & style of i18n labels