Toggle main window hide on tray icon click

• converted openBictoinAction to toggleHideAction
• put GUIUtil functions into a namespace instead of a class
• put window-related functions together in optionsdialog

Reasoning:

• toggle is more typical behaviour
• it's more functional
• better UX

The typical issue with toggling visibility is that when a window
is obscured by other windows but in the 'shown' state, hiding it
isn't what you want. I've added an 'isObscured' function to GUIUtil
that checks several pixels in the window to see if they are visible
on the desktop so that an obscured but shown window can be raised.

Conflicts:

src/qt/guiutil.cpp
src/qt/guiutil.h