Rearranged editor CSS styles and fixed some visual bugs
Fix mouse flickering between states when moving over input pane contents
Fix error with close command when no panes have focus
Simplify some logic and tweak comments in Editor.removePane()
Allow jumping relative to the end of the line with the jump command
Don't trigger the 'beforeExecuteCommand' event if the command is a no-op
Combine Editor commands with the CommandManager, and add several bugfixes

- All commands are now implemented through the global CommandManager
- The editor command bar now routes commands through the command manager
- Commands exposed to the user now define a template for matching user
  input. Arguments will be parsed and cast to the correct type when being
  passed to the command's function. Invalid input is detected
  automatically.
- Commands can now be called simultaneously by supplying independant
  promises to runCommand()
- When querying the editor for the focused pane, if no pane is currently
  focused the last focused input pane will be returned. Failing that, the
  last focused pane of any type will be returned.
- Editor commands are now defined with the rest of the editor extension,
  and take advantage of the new command manager API
Tweak link node animation styles and make link nexuses cover up the link lines of panes not connected to the nexus
Only call pane.focus() if necessary when adding a vertical split
Tidy comments
Fix jump command