Custom issue flagging

Description:
Linguist GUI notifies the translator of the state of a translation using a question-mark, check-mark, or exclamation point (and various colors)
However the exclamation point is only used for Qt Validation results, during the process of reviewing translations you have received you will find other reasons to indicate an problem with a translation. Currently the ability to communicate these problems with the translator is lacking.

Technical limitations:
currently the only data that lupdate doesn't overwrite is the translation data. even if extra-* meta-data were added it would not show up in the GUI and if that meta-data doesn't exist in the source code it will be overwritten the next time lupdate is run.

Proposed Solution:
support Customer Concerns (per translation) as part of the LS data format that lupdate doesn't overwrite.
when data exists for Customer Concerns, show the Customer Concerns text next to the translation and flag the translation as [exclamation point] Validation Failure.

this may be somewhat related to this other request - QTBUG-1436.

also: it seems like it would limit communication that there is only 1 translator note per message per language, rather than per translation if NumerousForm or LengthVariant are used.