Hide windows instead of closing them when clicking on close

[DanWin]

This implements event handlers to prevent the windows from being closed and hiding them instead. There is a difference between closing and hiding a window. When you close a window and open a project where this window is visible, it is empty, or in case of the FXMixerView, only displays the title bar. You can reproduce this by opening lmms, closing all windows and opening any project. All windows will be blank.

[Sti2nd]

What is the difference between hide and close?

Remember that pressing Ctrl + Tab let you select the different open windows, and pressing the key combo several times scroll through the open windows. I hope hiding and closing will be irrelevant for this (Qt) shortcut.

[DanWin]

It only affects the sub-windows in the QMidArea of LMMS, not the OS Window.
Close seams to do a bit more than only hiding a widget, which is what we want and is already done when clicking on the appropriate icon in the toolbar. For a more detailed discussion, see here: http://stackoverflow.com/questions/2293373/what-happens-when-i-call-qwidget-close

[Sti2nd]

not the OS Window.

That is Alt + Tab. Ctrl + Tab is reserved for programs. I just wanted you to make sure you can still use the shortcut. I am not technical enough, yet, to build my own builds.

[DanWin]

Oh, Ctrl not Alt. Thanks for the new shortcut. This shouldn't be affected. You can try it out by clicking on the icons on the top left to hide a window and show it again. It is working almost the same. The only difference is, that the window below is focused instead of no focus/focus on the closed window.