Open the main window when opening a second instance

[mkody]

Instead of throwing an error, launch the Assistant.

This way we can trigger it from the desktop or taskbar, which can be more accessible than a keyboard shortcut or double-clicking the tray icon.
Fixes #94

[imreallyliam]

Just had the privilege of building this on Fedora 34, works great with Wayland!

[Melvin-Abraham]

@mkody Thank you for the contribution 👋. I have made a minor change wherein the second instance error will be shown when user launches an instance in Dev Mode. This is done so as to make sure the developer is testing the code on a fresh instance. For end users launching the production version of the app, this won't be shown.

[mkody]

Thanks @Melvin-Abraham!

Just wondering if the if (!mainWindow.isVisible()) check you put should also be added when we use the tray context menu and the double-click even on the tray icon?

[Melvin-Abraham]

@mkody That particular check is not put there intentionally, since that option is for forcing the application to re-launch in case the app does not respond properly.