delete individual save slots
fix spacing
check before deleting save
buggy check file exists before option to delete
check if any saved info and prints instructions
Add new test_maps, which tests all reachable levels for referential integrity.

It starts in (hardcoded) `studyroom` and goes through all doors.
Checks performed (explicitly):
- door points to a valid door in a valid level
  Exception: doors can point to no door in other level, as in that case default position is used (position of door `main`); found in:
  - level _womens-bathroom_, door _secret_
  - level _valley_, unnamed door at [149,0]
  - level _black-caverns-2_, unnamed door at [133,7]
  - level _black-caverns-2_, door _seabluff_
  There are couple of doors pointing to _overworld_ (in _new-abedtown_), so there is some special treatment of it.
- door's property `button` has to be recognized by _inputcontroller_ (currently one of `UP`, `DOWN`, `LEFT`, `RIGHT`, `SELECT`, `START`, `JUMP`, `ATTACK`, `INTERACT`)
- door's property `sound` has to be `false`/`true` or name of valid sound
- door's property `key` has to be loadable key, can't test if the key is also placed somewhere in the world, because some keys are loaded dynamically (e.g. white_crystal)
- level's property `music` has to be name of valid music

Loading a level checks references to other things implicitly, e.g. tilesets images.

Level's field `map` is currently not needed for testing, so it's being set to `nil`, lowering memory consumption.

All levels are loaded first, then the checks are performed, so checking of doors doesn't cause multiple loads of the same level(s).
Also, this enables separating different tests (i.e. doors from music and possible future tests).

Another option would be to load a level, perform all possible checks immediately (except doors check), store information needed for doors check and load next level in the queue.
This should lower memory consumption even more, at the expense of new data structures.

Sounds and musics are being tested in batches - they are collected first and tested afterwards.
This improves error message in case of an error - all sources referring to failed sound/music are reported.
Fix inconsistency revealed by test_maps
makefile: add maps as prerequisite of test, needed for test_maps
test_maps: add checking of breakable_block's property `sound`
test_maps: don't print stack traceback on failure, only message
test_maps: modularize for correct test suite behavior, move `loadLevels()` from tests to `suite_setup`

Lunatest's `fail` and `assert_*` are not supported in `suite_setup`, replace with standard `error` and `assert`.
Rename tests; `_maps` infix is not needed, tests are identifiable through test suite name (`test_maps`)
test_maps: rename `getUserId` to `getSourceId`, fix typo
lunatest: Report skipped tests in case of failure of suite_setup
Merge pull request #2053 from AlfonZ42/test-maps

Testing maps for referential integrity
update newThread for love 0.9
update template conf
Merge pull request #2064 from DaNiwa/love-9-continued

Love 9 continued
Merge pull request #2031 from edisonout/one-singlular-sensation

Delete individual save slots
temporary fix
fix mixpanel