Add option to start minimized

[ConorIA]

Adds a checkbox to the 'Window' menu to start the app in the tray (i.e. skip main.show() on startup.)
This partially addresses #104.

I'm not sure what is going on in the Windows menu, so I didn't touch that, but I feel pretty confident about the code for macOS. Tried, tested and true on Linux, of course!

[shvchk]

Would be nice to also have this as an argument, like --minimized, so that Wire could be added to autostart to be launched minimized, and showed as usual if launched manually.

For now I'm using this script in autostart (on Linux) to start Wire minimized.

[ConorIA]

I'm not entirely sure what the --startup flag is for, but if it is true, I think the app window won't show. It is on the same line as the change I made in this PR. @lipis, is this used anywhere else in the app?

[ConorIA]

Come to think of it, my guess is that that flag is precisely for when you want Wire to launch on startup. 😌

[shvchk]

Ha! There really is if (!argv.startup) check before main.show(), thanks for the tip!

P.S. Then maybe it would be more clear to have both option and argument checked at once, like if (!argv.startup && !startInTray) instead of the second if (!startInTray) check?

[ConorIA]

Thanks for the suggestion. It's addressed in the last couple of commits.

[ConorIA]

Updated to include newer changes.

[lipis]

Handled in #329