Extending the linter

[Itms]

Following ​Phab:D213, I should extend the use of the linter for the following kinds of files :

• XML files can be checked for correctness with the XMLBear, using our RNG schemata. However we mustn't follow xmllint's recommendations for style which make our files unreadable

  XML can be formatted better.
  |----|    | /mnt/data/jenkins-phabricator/workspace/phabricator_lint/binaries/data/mods/public/gui/lobby/lobby.xml
  |    |++++| /mnt/data/jenkins-phabricator/workspace/phabricator_lint/binaries/data/mods/public/gui/lobby/lobby.xml
  |  24|  24| 
  |  25|  25| 		<!-- Left panel: Player list. -->
  |  26|  26| 		<object name="leftPanel" size="20 30 20% 100%-310">
  |  27|    |-			<object name="playersBox"
  |  28|    |-				style="ModernList"
  |  29|    |-				sprite_asc="ModernArrowDown"
  |  30|    |-				selected_column="name"
  |  31|    |-				selected_column_order="1"
  |  32|    |-				sprite_desc="ModernArrowUp"
  |  33|    |-				sprite_not_sorted="ModernNotSorted"
  |  34|    |-				type="olist"
  |  35|    |-				sortable="true"
  |  36|    |-				size="0 0 100% 100%"
  |  37|    |-				font="sans-bold-stroke-13"
  |  38|    |-			>
  |    |  27|+			<object name="playersBox" style="ModernList" sprite_asc="ModernArrowDown" selected_column="name" selected_column_order="1" sprite_desc="ModernArrowUp" sprite_not_sorted="ModernNotSorted" type="olist" sortable="true" size="0 0 100% 100%" font="sans-bold-stroke-13">
  |  39|  28| 				<column id="buddy" width="12"/>
  |  40|  29| 				<column id="status" width="26%">
  |  41|  30| 					<translatableAttribute id="heading">Status</translatableAttribute>
  

• Python files must be linted because we have very ugly Python code and bad habits for this language in general. But currently we don't have actual coding conventions, so we mustn't use an arbitrary linter configuration.
• Perl files could be linted as well.

[Itms]

In 20364:

Linting with Coala.

This new script and the associated configuration files lint our source code (JS and C++). This will be extended in the future, refs #4833.

Reviewed By: Imarok
Differential Revision: ​https://code.wildfiregames.com/D213

[Freagarach]

If we don't have Python Coding Conventions, we _can_ just use an arbitrary linter and set that as CC?