Fix translation lookup in KisActionRegistry

In b55fdf33, unknowingly for
translators, a default "action" message context was introduced for
.action files. However, this context was hardcoded, meaning that
translations would break if the text used a more specific disambiguation
context.

This is only part of the fix for bug 426992. To fix it completely, the
i18n team must start marking strings as verified -- translations marked
with 'fuzzy' are skipped by gettext.

CCBUG: 426992
CCMAIL: kimageshop@kde.org
(cherry picked from commit a48ec499)