- Stop attempting to start a world when no world's created/selected in server tab
- Better world's indexes handling between subgames lists

-> remember the previously loaded serverlist, and use if new one failed
-> show a "loading" serverlist

* abort if theres no active timer
* only reduce the timer.time of all timers when its necessary
* move updating timers_to_add into a seperate function

Now logs

ACTION[ServerThread]: player sets time to 6:03

instead of

ACTION[ServerThread]: player sets time to 6:3

  * The time command now accepts parameters in the form <hour>:<minute>,
    and if invoked with no parameters returns the current time in said format.

Items dropped into unloaded map space will crash game so here's a fix...

Don't ident too much, and add a comment.

On calling clear_redistered_biomes the registered_biomes table is cleared
by creating a new empty table, but the pointer is not updated to point to
the new one. So after calling more register_biome, the registered_biome
table always contains 0 items, which is an error. Instead, the table is
cleared by removing all its items so the pointer (minetest.registered_*)
remains valid.

As the fly mode option is avaiable in-game, this is not used anymore.

This fixes main menu not fitting for 800x600 (and lower resolutions) on PC builds

Create Parameters on world initialisation and set settings of old worlds

Fixes #2497.

Fixes #2037

Client now informs about incompatible servers from the list, this permits to prevent the protocol movements.
Server announces its supported protocol versions to master server

Fix unused (and so, broken) enable_rollback_recording. This option must be reloaded at server loop but loaded when server starts, for data consistency (not a hot load variable)
ok @ShadowNinja