Use Int instead of QString in TableRow

The main purpose of this patch is to reduce memory and cpu usage while
rendering list table.
It has been done through:

1. use of QLatin1String where applicable,
2. moving i18nHeaders to function, because not all headers are needed

every time table is rendered,

3. moving column type detection to function, because of the same reason

as above,

4. using Int instead of QString as key in TableRow, because it's less

error prone, faster, and smaller,

5. applying multiarg instead of arg chaining, as it doesn't create

temporary strings.

Differential Revision: https://phabricator.kde.org/D6166

Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>