Improve save-related actions

Fixes #67 (closed), depends on !38 (merged).

This MR tries to clarify a bit the way the save-related actions work, by connecting in particular some signals so that actions and widgets are properly updated on file status change.

This made me realized that the "Revert" action was actually a "Reload" action when the file wasn't modified, in particular in its initial state. So I renamed the corresponding menubar and toolbar items accordingly (with icon and tooltip), and I connected them to the "modified-changed" signal to switch to "Revert" when the file is modified.