Display only icons in column "Translation Status"

Summary:
catalogmodel: Convert enum CatalogModelColumns to enum class

catalogmodel: Factorize calculation of translation status into CatalogTreeModel::getTranslationStatus()

catalogmodel: Display only icons in column "Translation Status"

This lets the user save some screen space by shrinking this column.

catalogmodel: Fix compiler warnings about incomplete switch operator

screenshot: {F6886384}

Test Plan:
Manually tested:
 1. contents of column "Translation Status" with Russian digikam.po where we have all 3 statuses;
 2. new tooltips;
 3. sorting by column "Translation Status".

Reviewers: sdepiets, aacid

Reviewed By: sdepiets

Subscribers: kde-i18n-doc

Differential Revision: https://phabricator.kde.org/D21777