Plugin manager

[jlap]

The current way to manage plugins is to add or delete plugins from the plugins folder. Whilst this works well, I'd rather check or uncheck them in a friendly interface.

It really shouldn't be a lot of work but it would be a great improvement.

[jlap]

This would allow the adding of hooks for installing and uninstalling plugins. That is more and more necessary.

Example, mapcache plugin ( #75 ) needs a new table in the database. Every time it is called, it tries to create the table if it doesn't exist. There is no way currently to remove that table afterwards.

A simple plugin management system would help a lot for this kind of problems.