Faster big projects and the .qetz format: three PRs to try

[ispyisail]

Big projects are slow to open and save because QElectroTech builds every folio before it shows the first one: 29 s to open a 136-folio project, 33 s to save it after one change. Three PRs now fix that, in steps that can be reviewed and merged one at a time.

PR	What a user gets	Changes the saved file?
#1461	Folios built only when shown. Off unless QET_LAZY_FOLIOS=1 is set	No
#1462	.qetz (a zip with a database inside) as a choice in Save As. .qet stays the default	Only if chosen
#1463	.qetz by default, the database as the source of truth, folios built when needed for everyone. The .qet is kept beside it, never overwritten	Yes

With all three on the 136-folio project, it opens in 1.8 s, saves in 4.75 s, and the file is 3.2 MB instead of 7 MB.

Each was checked on 342 real projects from the forum, the bugtracker and GitHub, written by QElectroTech 0.3 to 0.200: the results are identical to master's.

#1463 is the proposal for the questions in #1440; nothing there is decided yet. Merge order: #1461 and #1462 first, in either order, then #1463.