Ubuntu: fix insecure access to CWD when looking up files

The logic wa sapparently to make sure that we can find the QML file when run
straight from the build dir. However, it is important to use the path to the
aplication binary instead of the $CWD because otherwise Trojita would do
extremely dumb thing when called from inside an attacker-controlled directory.

This commit also adds the path to the app's exec file to the list of paths to
perform the lookup in order to preserve the desired functionality.
Ubuntu: clean up the path manipulation a bit

There's no need to clutter the original variable.

Also use QFile::exists as per Thomas' recommendation.
Ubuntu: do not mess with appDir path when using user-provided location

This enables stuff like `trojita --qml /path/to/it`.
Ubuntu: remove support for passing paths to QML files

There's no need to do this. The app will always load the QML from a path
relative to the application's own location (which handles the case for running
from inside the build directory). If that is not available, systemwide locations
are consulted as a fallback.

I just don't see any use case behind having to pass paths here.
Ubuntu: Remove debugging output
Ubuntu: It's Trojitá, not Trojita, and add l10n
Merge branch 'ubuntu-qml-path'

REVIEW: 117205