The showSystemTrayIcon's default value is false on linux and else true.

Hi,
there was a little discussion on https://github.com/tux3/qTox/pull/756 (https://github.com/dubslow/qTox/commit/fcc8257924ee5340677a60b82c8678261b020ec3) and a think I found a reasonable soulution for both parties, see the code below:
```c++
#ifdef Q_OS_LINUX
#define SHOW_SYSTEM_TRAY_DEFAULT (bool) false
#else   // OS is not linux
#define SHOW_SYSTEM_TRAY_DEFAULT (bool) true
#endif
```
```c++
showSystemTray = s.value("showSystemTray", SHOW_SYSTEM_TRAY_DEFAULT).toBool();
```
I found a fix on http://stackoverflow.com/questions/17193307/qt-systray-icon-appears-next-to-launcher-on-ubuntu-instead-of-on-the-panel. It is not hard to implement, but we'll depend on gtk.
What do you think about it?