fixtureeditor: reject definitions without modes

[gmint]

Summary

Prevent the Fixture Editor from saving a fixture definition that has no modes.

Root cause

Since v4.14.4, QLCFixtureDef::loadXML() correctly rejects definitions without modes to prevent the crash fixed by #1775. The Fixture Editor could still create and save exactly that invalid definition, leaving it impossible to reopen.

Changes

• validate that a definition contains at least one mode before Save or Save As
• show the existing missing-information warning, select the Modes tab by widget reference, and focus the add-mode control
• rename the validation helper to reflect its broader responsibility

This preserves the engine-side load guard while preventing the editor from producing files it cannot reload.

Fixes #2095.

Validation

• git diff --check
• inspected against the v4.14.4 source and the reported no-mode QXF reproduction
• the upstream CI build is awaiting maintainer approval because this PR originates from a fork

The local workspace does not include CMake or Qt development dependencies, so the project's make check suite could not be run there.