logic: background indexing (issue #175)

* allow hiding of indexing dialogs to browse loaded project while indexing
* show indexing status in status bar while indexing
* use different dialog views for different use cases to avoid state collisions
* fixed error label in status bar jittering while indexing
* show hand cursors for clickable elements in status bar
* indexing is not interrupted on pressing ESC while in hidden
* disable error table and error label while indexing
* refactored indexing messages
* forbid indexing and changing project while indexing
* don't show indexing status messages in status bar when indexing in the background
* allow tasks to run in the background

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src/lib_gui/qt/window/QtWindowStack.cpp

@@ -13,7 +13,7 @@ QtWindowStack::QtWindowStack(QObject* parent)
 {
 }
 
-QtWindowStackElement* QtWindowStack::getTopWindow()
+QtWindowStackElement* QtWindowStack::getTopWindow() const
 {
 	if (m_stack.size())
 	{
@@ -23,7 +23,7 @@ QtWindowStackElement* QtWindowStack::getTopWindow()
 	return nullptr;
 }
 
-QtWindowStackElement* QtWindowStack::getBottomWindow()
+QtWindowStackElement* QtWindowStack::getBottomWindow() const
 {
 	if (m_stack.size())
 	{
@@ -33,7 +33,7 @@ QtWindowStackElement* QtWindowStack::getBottomWindow()
 	return nullptr;
 }
 
-size_t QtWindowStack::getWindowCount()
+size_t QtWindowStack::getWindowCount() const
 {
 	return m_stack.size();
 }