Add settings editor commands to focus next and previous settings

Issue Type: <b>Bug</b> 1) When the cursor in the search box, <kbd>DownArrow</kbd> moves the keyboard focus to the list of settings. However, <kbd>UpArrow</kbd> does not bring me back to the search box. 2) When the keyboard focus in the TOC, <kbd>RightArrow</kbd> should move the keyboard focus to the closest item in the list of settings. 3) When the keyboard focus is in the list of settings, <kbd>LeftArrow</kbd> should move the keyboard focus to the highlighted node in the TOC. 4) It seems unusal that the section headers and subheaders in the list of settings can take the keyboard focus. It would feel more natural if the headers and subheaders would be skipped. 5) When a settings cells has the keyboard focus, <kbd>Enter</kbd> should move the keyboard focus to the value editor (same as <kbd>Tab</kbd>. <kbd>Shift+Tab</kbd> and <kbd>ESC</kbd> remain to remove it back to the settings row. VS Code version: Code - Insiders 1.25.0-insider (5d6156a0f8b68fe7e9429facbcfaa7c061a8b3e3, 2018-06-25T09:32:08.937Z) OS version: Darwin x64 16.7.0 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz (8 x 2500)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>multiple_raster_threads: enabled_on<br>native_gpu_memory_buffers: enabled<br>rasterization: enabled<br>video_decode: enabled<br>video_encode: enabled<br>vpx_decode: enabled<br>webgl: enabled<br>webgl2: enabled| |Load (avg)|2, 2, 2| |Memory (System)|16.00GB (1.06GB free)| |Process Argv|/Users/kmaetzel/Applications/Visual Studio Code - Insiders.app/Contents/MacOS/Electron -psn_0_17449123| |Screen Reader|no| |VM|0%| </details> <!-- generated by issue reporter -->