Use initializer list constructor for std::map (C++11)
Add “find bundle for new items” API

We need this when installing standalone bundle items, saving macros, and when creating items in the bundle editor without a bundle selected.

The user experience of this code is however rather poor in that you can’t actually crete new bundles, the dialog text is not specific, and the logic for what bundle to preselect can be improved (prefer previously created bundle falling back on a local bundle, if any).
Implement Save Macro functionality

Closes #459.
Add Edit → Macros → Save Macro… (⌃⌘M)
Treat bundle items without path as modified

This allows us to create an in-memory bundle item, reveal in bundle editor, and have the bundle editor warn if the user doesn’t save.