hide/show message pane
Fix coding style
Make the "one thing at time" layout better fit with the rest of the code
GUI: don't auto-advance to the message view when within the one-at-time layout
GUI: don't switch the layout too early

If the stored user's preferences were applied too early, the order of the
toolbar icons would not be preserved because the one-at-time layout manipulates
the toolbar.
GUI: fix the icon for the "go back" button in the one-at-time layout
Use QStackedWidget instead of manual widget showing/hiding

Thanks to Thomas Lübking for reminding me that the widgets have to be explicitly
shown after reparenting.
GUI: disable the wide mode on small screens

Suggested by Thomas Lübking.
GUI: wait a bit before reacting to screen resolution changes
GUI: enable sizes and positions of various widgets

Fully separate settings are maintained for the wide and compact layout modes.
Nothing is saved/restored when in the one-at-time mode.
GUI: also save & restore the sizes and state of the header fields in the message list
GUI: keep the actions for disabling/enabling headers in action with their actual state when restoring
GUI: resize the header columns *initially*, but respect the user preferences later on
GUI: listen to QDesktopWidget::workAreaResized instead of just resized()

Sugegsted by Thomas Lübking.
Merge branch 'very-compact-layout'

Conflicts:
	src/Common/SettingsNames.cpp
	src/Gui/Window.cpp
	src/Gui/Window.h
REVIEW: 110653
fixes #40
fixes #564