Add basic unit tests for MainWindow

[ddennedy]

No description provided.

[Copilot]

Pull request overview

Adds an initial CTest/QtTest-based harness to exercise basic MainWindow invariants, and refactors the build so the application’s compilation units can be reused by test executables.

Changes:

• Introduces a QtTest-based TestMainWindow test binary and registers it with CTest (offscreen platform).
• Adds a top-level tests/ CMake subtree and enables CTest integration.
• Refactors src/CMakeLists.txt to build core sources as an OBJECT library (shotcut_objects) and links the shotcut executable (and tests) against it.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/mainwindow/mainwindowtest.cpp	Adds QtTest cases covering basic MainWindow state, singleton access, action registration, and a profileChanged signal check.
tests/mainwindow/CMakeLists.txt	Builds and registers test_mainwindow with CTest and sets QT_QPA_PLATFORM=offscreen.
tests/CMakeLists.txt	Adds the mainwindow tests subdirectory.
src/CMakeLists.txt	Splits app sources into shotcut_objects and links shotcut against it to enable reuse by tests.
CMakeLists.txt	Enables testing and adds the tests/ subdirectory at the top level.