Add stub for window management.
Fix `resizeWindow` signature.
Updated window management tests.
Remove `DO_DISABLE_SPECIFIC_TEST_SUITES_FOR_TRAVIS_CI`.
MAINT: remove deleteLater() method upon closing window.

Indeed QWidget are set to be deleted upon closing.
Fix memory leak in `GraphicsView` class.
Fix potential segmentation fault in `test_window_management.cpp`.
Fix memory leak when the user forgets to close windows.
Refactor functions that create windows.
Remove commented lines.
Fix implementation of window creation functions.
Reformatted member attribute declaration in `GraphicsView` class.
Fix indentation issues.
Temporarily remove asserts checking if windows are closed.
Add TODO comment in the `test_window_management.cpp`.
Run tests with verbose output for easy troubleshooting.
Add temporary verbose comments to troubleshoot in Travis CI.
Temporarily disable tests to check there is no segfault in Travis CI.
Merge branch 'test-window-management' of https://github.com/DO-CV/DO-CV into test-window-management
Renamed redundant test cases.