#315 Categories for %TODO commands

[Tim Hoffmann]

from https://sourceforge.net/projects/texstudio/forums/forum/907840/topic/5473775

Allow for different categories in %TODO commands, e.g.
%TODO ASK:
%TODO RESEARCH:
and group them by their category in the structure view.

One could also investigate, if latex todo packages (easy-todo, todo, todonotes) should be supported and if they provide categorization.

[Tim Hoffmann]

From hg 5150 (5906006d023f) on, arbitrary %TODO-style comments are accepted (well, any capitalized word, % FIXME, % ASK). Additionally, a single space is permitted between the comment char and the word.

Note: Basic support for \todo-style commands has been added quite a while ago.

Grouping or dedicated view is reqested separately in [feature-requests:#418]