add view filtering

the message view would gain a bar with a line edit for an incremental text filter and a set of options, like the usual "case insensitive", "whole words only", "regular expression", restricting searched fields, but also message states like "not validated" and "unfinished".

the context view could have a much more limited filter bar as well. to be considered.

an example use would be "regular expression '.+' in translation field, unfinished only" to show only started but not yet finished translations.