central logging mechanism

[splitbrain]

This introduces a logger mechanism and a new data/log directory. This is a first proof of concept. It's built on top of the new global error handling mechanism of #3203

Things to discuss:

• should we adopt PSR-3 somehow? if yes, how and should plugins be able to drop-in other psr3 loggers?
• how to configure which facilities shall be logged?
• should we implement a log deletion feature?
• is the log format sensible?
• should we implement a log viewer admin plugin?
• should logging trigger events (it should maybe replace the deprecation event)

[splitbrain]

I worked some more on this. Let's summarize:

• There \dokuwiki\Logger now which is a singleton (per logging facility) that can be instanced using \dokuwiki\Logger::getInstance($facility)
• There are three default facilities: error, debug and deprecated but devs can add new facilities as they like (simply pass a string as facility)
• static convenience methods are available. Eg. \dokuwiki\Logger::error() to directly log to the error log
• logs are stored in data/log/$facility/$date.log
• There is a new logviewer plugin bundled which allows to read the logs from the admin menu
• There is a new config option to disable logging facilities (debug is disabled by default)

Things I'm unsure about:

• I think different logging facilities are more useful than traditional log levels, thus implementing PSR-3 might not be worth it
• do we want to encourage plugin authors to use their own facility?
  • if so, the interface of the log viewer is probably not suitable
  • we might need to inverse the log setting, from disabling to explicitly enabling
• stuff from the original post

Feedback welcome

[splitbrain]

merged into master