COMMON: Add FilePath::createDirectories()
COMMON: Add FilePath::getConfigDirectory()
COMMON: Use FilePath::getConfigDirectory() in ConfigMan

Note that with change, the location of the config file has
changed for both Mac OS X and GNU/Linux (and other Unixoids).

- On GNU/Linux, the file is now $XDG_CONFIG_HOME/xoreos/xoreos.conf
  (previously: $XDG_CONFIG_HOME/.xoreosrc)
- On Mac OS X, the file is now $HOME/Library/Preferences/xoreos/xoreos.conf
  (previously: $HOME/Library/Preferences/xoreos\ Preferences)
COMMON: Add FilePath::getUserDataDirectory()

This is where user data like a default log or game saves
will be put in the future.

- On GNU/Linux, this will evaluate to $XDG_DATA_HOME/xoreos/
- On Mac OS X, this will evaluate to $HOME/Library/Application\ Support/xoreos/
- On Windows, this will be the same place as getConfigDirectory()
MAIN: Open log file after target has been identified

So that each target can define its own log file.
COMMON: Add DebugManager::getDefaultLogFile()

Returns the path of where the log file is written to by default.
MAIN: Always open a log file by default

Override by setting logfile to an empty string or
setting nologfile to true.
MAIN: Document --nologfile