Switch less often to edit mode

This change
* Removes IMode::type, ModeManager::activateModeType, and
  IEditor::preferredModeType, and adds IEditor::isDesignModePreferred
  instead
* Adapts the mode switching code in EditorManager to handle multiple
  windows, for example switching to edit mode should only happen if
  the editor/view is in the main window. Otherwise the editor window
  should be raised and focused
* Renames EditorManager::NoActivate --> DoNotChangeCurrentEditor
* Reverts the EditorManager::ModeSwitch logic to switch mode or
  make the current editor visible by default, introducing
  DoNotMakeVisible flag instead
* Fixes a few instances where EditorManager::ModeSwitch should have been
  used

One non-trivial problem left: If you open a .ui file and switch to an
external editor window, edit mode is activated, because the current
editor no longer is a .ui file, which means that the design mode gets
deactivated.

Change-Id: I76c5c2391eb4090143b778fb103acff3a5a1ff41
Reviewed-by: David Schulz <david.schulz@digia.com>

src/plugins/coreplugin/editmode.cpp

@@ -55,7 +55,6 @@ EditMode::EditMode() :
     setIcon(QIcon(QLatin1String(":/fancyactionbar/images/mode_Edit.png")));
     setPriority(Constants::P_MODE_EDIT);
     setId(Constants::MODE_EDIT);
-    setType(Constants::MODE_EDIT_TYPE);
 
     m_rightSplitWidgetLayout->setSpacing(0);
     m_rightSplitWidgetLayout->setMargin(0);