fix(web): restore `dragEnd` function 🔥

Relates to #5947.

I resolved a merge conflict in #5947 in the wrong direction and
accidentally deleted the `dragEnd` function. This restores it.

chore(developer): move kcframe into its own folder

fix(developer): kmdecomp string overflow

fix(developer): render OSK nicely on touch devices

fix(web): support saving focus for custom OSK interactions

fix(web): improve keyboard switch performance

fix(web): remove obsolete popupBaseTarget

auto: increment master version to 15.0.157

fix(windows): fix loadkeyboardoptions core memory error

Picked up in development of #5963.

Relates to #3621.

Adds `readonly` group support and verifies that keyboards meet
constraints around usage of these groups.

(Note: fixes test_valid.kps and test.bat which seemed to have some
invalid tests?)

auto: increment master version to 15.0.156

fix(web): clear longpress timeout if user does a flick up

fix(web): shorten setFocusTimer()

fix(web): reset scroll anchor on touchend

Relates to #3621.

Add support for `&layerChanged` system store. This store is set to `1`
before a `begin postKeystroke`, if the keystroke it follows resulted in
a layer change, either programatically through a keyboard rule, or
through a `nextlayer` property of the touched key.

Relates to #3621.

This adds support for `begin postKeystroke` to KeymanWeb. This is now
close to final, apart from support for `&layerChanged`.

Relates to #3621.

Adds basic support for `begin newContext`.

Relates to #3621.

This adds basic compiler support for the `begin newContext` and
`begin postKeystroke` statements, along with the corresponding `gn` and
`gpk` entry points in the compiled JavaScript keyboard.

TODO: `readonly` semantics, usage constraints, `&layerChanged`.

Relates to #3621.

Initial draft of keyboard to test start-of-sentence detection.

Co-authored-by: Marc Durdin <marc@durdin.net>

auto: increment master version to 15.0.155

chore(linux): Run ibus-keyman tests as part of the build 🎫

Part of #5613.

Array has to be NULL terminated.

Part of #5613.

chore(linux): Remove unused test methods 🎫