Add test play mode to the editor

[Forestidia86]

When testing freshly build levels you have to save, go out of the editor and play the build level, which is rather tedious.
Furthermore with normal play all your solutions during testing get auto saved by default what e.g. Ramond finds annoying since it can clutter your replay folder.

Therefore a test play mode from inside the editor seems to be a reasonable feature.

(Corresponding discussion from IRC:
[17:10:03]    <Ramond__> one other thing maybe, while testing your own level the game will auto-save each of your successful attempts, it will clutter after a while
[...]
[18:25:10]    <Forestidia> The auto save feature is important.
[18:25:24]    <Forestidia> It can be turned off I think.
[18:26:14]    <Ramond__> yeah it is important for playing, but rather hindering during level testing
[18:26:33]    <Forestidia> But what is your suggestion?
[18:27:06]    <Ramond__> I don't really have any proper suggestion, it only felt a bit annoying to having to clean out the replays manually
[18:27:15]    <Ramond__> maybe someone else has a good idea
[18:27:16]    <Forestidia> okay
[18:27:44]    <Forestidia> You can Esc the level before you hit the save requirement.
[18:27:58]    <Forestidia> I often do that when testing.
[18:29:02]    <Ramond__> that's a possibility but still a slightly tedious workaround (always having to have that in mind)
[18:29:27]    <Forestidia> The thing is to have a replay for the final version is important for coverage.
[18:30:08]    <Ramond__> dunno if feasible but what if there's a way to directly go into a 'test play' mode from the editor (skipping going back to menu to select play) which is identical to the normal play mode except it won't auto-save? (can still be saved manually somehow)
[18:30:49]    <Forestidia> A test-play mode from inside the editor would be good for other reasons as well.
[18:31:28]    <Forestidia> Since it's then generally less tedious to test your level.
[18:31:44]    <Ramond__> yes
[18:31:51]    <Ramond__> though for some reason I thought it was intentionally left out
[18:32:14]    <Forestidia> I don't think so actually.
[18:32:27]    <Forestidia> But I don't know for sure.)

[Simon]

Right, this might be nice. Made github issue: #378 Editor: Add playtesting mode.

Maybe even with the possibility to spawn loose lixes on demand anywhere in the level during test play.

Feels like a more complex feature. But the old save-exit-play-exit-edit-cycle feels unntaural, too.

-- Simon