Add an explicit config to control enabled dictionaries

[kanru]

Is your feature request related to a problem? Please describe.

Some dictionaries like the alternative one should be optional.

Describe the solution you'd like

Add a config option that is a comma separated list of dictionaries file names enabled.

All search paths should be scanned and the first match is used.

Priority order

• $XDG_DATA/chewing/dictionaries.d
• $XDG_DATA/chewing
• $HOME/.chewing
• $CHEWING_SYS_PATH/dictionaries.d
• $CHEWING_SYS_PATH

Describe alternatives you've considered

Load them all, use separate dictionary to control enabled or disabled list.

Cons

• involves filesystem operations
• sorting by dirent is not very portable

Additional context

Chewing preferences should expose an UI to enable, disable, and sort the dictionary.