Add "Collapse parent tree" command as a keyboard shortcut slot

[piroor]

It should do:

1. Activate the parent tab.
2. Collapse the tree.

Inspired from piroor/treestyletab#3419

[piroor]

A new keyboard shortcut will be added here:

tst-more-tree-commands/manifest.json

Lines 48 to 49 in c18b790

	}
	},

tst-more-tree-commands/background/background.js

Lines 354 to 355 in c18b790

	return;
	}

Codes for a new menu command will be added here:

tst-more-tree-commands/background/background.js

Lines 155 to 156 in c18b790

	}
	};

tst-more-tree-commands/background/background.js

Lines 225 to 226 in c18b790

	default:

tst-more-tree-commands/background/background.js

Lines 284 to 285 in c18b790

	default: