[ENH] Corpus: add title attribute selection dropdown

[PrimozGodec]

Issue

When the corpus is loaded from a file or made out of the data table, the title attribute is not set if it does not exist in data already.

Description of changes

With this PR we:

• implement the dropdown to select the title attribute
• implement the strategy to select the most suitable title attribute by default
• add unit-tests

When this PR is checked I will update the documentation since new images must be made.
This PR also does not address the numbering of documents in a Corpus Viewer it will be solved in a separate pull request #482.

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

If I select grimm-tales-selected and change the Title attribute to 'no title', I don't think the titles become the default, but the old title property is still used.

[codecov-io]

Codecov Report

Merging #481 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
+ Coverage    85.7%   85.77%   +0.06%     
==========================================
  Files          34       34              
  Lines        2001     1996       -5     
  Branches      336      333       -3     
==========================================
- Hits         1715     1712       -3     
- Misses        247      248       +1     
+ Partials       39       36       -3

[PrimozGodec]

@ajdapretnar another heuristic was used in the corpus that outputted the title attribute even it was not marked in a table. In case that there was no title attribute marked in a table and table had one of the attributes "title", "heading", "filename", it was decided as a title. Since we are building a new strategy for deciding on a title attribute, I removed current heuristics from the corpus. Anyway, since the strategy was good I merged it in a corpus widget for deciding on a default title attribute.