Add the feature to merge projects

[raspopov]

• 💡 Added the feature to select multiple files when opening. The first file becomes the main project, and the others are merged into it.
• 💡 Added the feature to drag and drop project files into the program window, with a prompt to merge them.
• 🐞 Fixed a bug where a modified project was overwritten without a save prompt when opening a new file in MainWindow::on_actionOpenProject_triggered().
• 🐞 Fixed unnecessary project modifications when any window is normally activated in MainWindow::on_mdiSubWindowActivated().

Changes to the codebase:

• Added the AppProject::filePath() method, which returns the file path to the project, replacing the removed MainWindow::_projectFilePath class member that duplicated this functionality.
• All functionality related to closing a project has been moved from the AppProject::loadProject() method to the AppProject::closeProject() method.
• The AppProject::saveProject() method now remembers the save directory on its own; previously, all calls to saveProject() were paired with setSavePath().
• Added the MainWindow::closeProject() method—removed duplicate code for closing the project and prompting to save changes.
• The MainWindow::dragEnterEvent() and MainWindow::dropEvent() methods have been added to implement file dragging.
• The signature of the MainWindow::loadProject() function has been extended to support adding multiple files to a single project and handling user-initiated cancellation of the operation.
• The MainWindow::saveProject() method now maintains its own list of recent files using addRecentProject().

Fixes #121.