Add new function Common::writablePath() into Path.cpp

Qt methods QStandardPaths::writableLocation and QDesktopServices::storageLocation
using QCoreApplication::applicationName and QCoreApplication::organizationName
which contains incorrect data when Trojita code is used by another Qt application.

This patch wrap QStandardPaths/QDesktopServices calls with setting correct
application data into safe function Common::writablePath().

REVIEW: 112227
Use a real iteration here, not manually unrolled loop

CCMAIL: pali.rohar@gmail.com
Make sure that both MainWindow and ComposeWidget use the same path for draft storage

There are two types of drafts, one is for the explicitly saved ones, the other
for auto save. The auto save shall function without being visible to the user,
so saving them under a non-i18n directory is fine. The explicitly saved ones,
though, are somehow expected to be visible to the user, so using i18n here makes
sense.

At least that's my idea -- it would IMHO be equally OK to just dump them into
paths without any i18n at all, but that's not how this code was originally
written.

CCMAIL: thomas.luebking@gmail.com