qml: Compile application sources ahead of time

[johnny9]

This compiles the application QML and JavaScript with qt6_add_qml_module.
qmlcachegen now checks these files during the build and embeds the generated
units in the executable. Existing qrc:/qml/... paths remain unchanged. Images,
fonts, and controls/qmldir remain in the manual resource file.

On Qt 6.6 and newer, the application uses only the embedded AOT units. It does
not read or write host-generated .qmlc or .jsc files. Qt 6.2 through 6.5
cannot control embedded and host cache files separately, so those versions use
the embedded QML source with the host cache disabled.

This prevents invalid QML from reaching packaged builds and prevents stale host
cache files from affecting application upgrades.

The change also adds unit and launch tests for the cache policy and adds the Qt
private declarative headers required by cachegen builds.

Tested on Ubuntu 24.04 with Qt 6.4.2:

• Built bitcoin-core-app, bitcoinqml_unit_tests, and bitcoinqml_qmltests.
• Passed both CTest targets.
• Passed the cold-launch and stale-cache smoke test.
• Confirmed that an invalid QML token fails the build in qmlcachegen.
• Passed git diff --check.

Fixes #695
Fixes #854