Standardized the line separator to LF.
Converted history.py into a package, moved select_commit.py into it.
Various history-related features, fixes and refactors.

- Implemented branch selection for history viewers.
- Implemented a dialog for picking one or more branches.
- Bundled all log filtering controls into LogFilter widget.
- Refactored LogGraph to get the log directly from the Repo.
Extracted some dialog handling code into berk.gui.Dialog class.
Fixed the bug that didn't update file statuses, just the icon.