Use new format for “installed_locales”

Since commit e862e7cbb64faeb554eb8eac2e60efd78a9631ad, the
“installed_locales” entry in the $_APP “core” section no longer has keys;
its data has been moved to the values in the array. This changes the
lookup code accessing that entry.
Move 2nd half of QlApplication::load_section() to merge_section()

Further improve on commit 1621715a64a0c3e84ae83ca8379d409828efcf62, so now
the caller of load_section() can modify the just-loaded section before
it’s merged into $_APP.

Also fix access level of QlApplication member variables, improve comments,
and add some locks-related logging.
Add __pycache__ to .gitignore