systray: restore original behaviour for systrayicon check

In response to an issue that would cause a lost window when minimizing to tray on a system without a systray, commit 316750ad introduced a bug for versions of Qt that would return null icon geometry even when the icon was visible. Subsequently, this was byspassed with commit f25e66aa which only applied to GNU/Linux systems, leaving the hack inplace for Windows and macOS where the bug had not originally occured.

This commit:
- makes the "MinimizeOnClose" option always visible
- uses Qt's built-in method for "isSystemTrayIconVisible"
- changes the default "MinimizeOnClose" setting to true

Gitlab: #1623
Change-Id: I3b99c7fb952eedea63ae9c12d207ceb3c9bd4988