frontend: Fix exit from tray context menu

[zorbathut]

Description

Due to unintuitive Qt behavior in Wayland, right-clicking tray context menu and choosing "exit" works only if the main window has already been displayed.

Work around by sending the close event manually when necessary.

Fixes #13494.

Motivation and Context

--minimize-to-tray allows starting in tray-minimized form. On Wayland, this prevents you from using the right-click menu to close it.

How Has This Been Tested?

Opened with --minimize-to-tray, verified I could close it. Opened without that, verified I could still close it. Opened with it and opened the window manually, verified I could still close it.

Tested on Manjaro Linux, Wayland, KDE desktop.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.