refactor: Menus, Toolbars and Actions in MainWindow

Partly reorganized code in MainWindow related to generation of menus, toolbars and the actions behind it. Also fixed some minor issues. 

- Removed qttools.Menu class, which was a workaround to offer tooltips for menu entries. Qt 5.1 is able to do it itself.
- Dropdown menus for toolbar buttons "Take snapshot" (main toolbar) and "Restore" (file view toolbar) repaired and working now.
- Removed the Help button from main toolbar without replacement. All entries available in the main menu.

.gitignore

@@ -26,7 +26,7 @@ debian/backintime-qt
 # compressed files
 *.gz
 
-# Translations
+# Compiled translations
 *.mo
 
 # Makefile
@@ -36,13 +36,16 @@ qt/Makefile
 # coveralls.io status
 .coverage
 
+# coverage result files (raw data and reports)
+# .coverage  # already ignored, see above
+**/htmlcov/**
+
 # sphinx doc build
 common/doc-dev/_build/doctrees
 common/doc-dev/_build/html
 
 # PyCharm IDE project settings
 .idea
 
-# coverage result files (raw data and reports)
-# .coverage  # already ignored, see above
-**/htmlcov/**
+# Linter configuration
+.flake8