Keyboard shortcut to focus a specific pane

[aolo2]

The TAB-based focus system is currently working alright, but with the growing number of windows/panes, cycling between them might start to take very long. I propose a shortcut navigation system, which focuses a specific pane with a key combination (e.g. Alt+1 for source, Alt+2 for watches etc). As for TAB, it should probably only cycle inside the focused pane.

[nakst]

As discussed in issue #25, I don't want to have many debug windows that take input focus. I'd rather have as much as possible be controlled from the console.

Although, it would be nice to have keyboard shortcuts for switching between tabs of display-only windows.

[nakst]

Added in 4f1724b.

Keyboard shortcuts must be manually assigned, as detailed in https://github.com/nakst/gf#custom-keyboard-shortcuts.