Add gutter to Command Palette to avoid overlapping with scrollbar (#6965

)

The command palette has some content that can overlap with its
scrollbar.  This PR adds a 16px gutter for the scrollbar, as recommended
[here](https://docs.microsoft.com/en-us/windows/uwp/design/controls-and-patterns/scroll-controls#:~:text=leave%2016px%20of%20padding%20on%20the%20edge%20of%20the%20viewport).

## Detailed Description of the Pull Request / Additional comments
You can repro the overlap in the default configuration by grabbing the
scrollbar with the mouse pointer.  But there's an accessibility option
that makes this more obvious: Settings > Display > Automatically hide
scroll bars.  With that option enabled, the text is _always_
overlapping.

The gutter does look slightly larger than it needs to be when the
scrollbar is thin or completely hidden.  Dynamic reflow may help, but
unfortunately, I don't know enough XAML to wire that up.  MUX has a
promising visual state named `ScrollBarsSeparatorExpanded`, so the
scientists suggest we _could_, while the designers are still pondering
whether we _should_.

## Validation Steps Performed

Old appearance:
![image](https://user-images.githubusercontent.com/10259764/87817879-94d85100-c81e-11ea-956c-ca0e23576fef.png)

New appearance with fat scrollbars:
![image](https://user-images.githubusercontent.com/10259764/87817914-a4579a00-c81e-11ea-9e9d-195969e6da95.png)

New appearance with thin scrollbars:
![image](https://user-images.githubusercontent.com/10259764/87818061-dff26400-c81e-11ea-866e-088f72276343.png)

New appearance with no scrollbar:
![image](https://user-images.githubusercontent.com/10259764/87819674-7758b680-c821-11ea-98b7-dddd1573c242.png)

src/cascadia/TerminalApp/CommandPalette.xaml

@@ -195,9 +195,10 @@ the MIT License. See LICENSE in the project root for license information. -->
                         <Grid HorizontalAlignment="Stretch"  >
 
                             <Grid.ColumnDefinitions>
-                                <ColumnDefinition Width="16"/>
-                                <ColumnDefinition Width="Auto"/>
-                                <ColumnDefinition Width="*"/>
+                                <ColumnDefinition Width="16"/>   <!-- icon -->
+                                <ColumnDefinition Width="Auto"/> <!-- command label -->
+                                <ColumnDefinition Width="*"/>    <!-- key chord -->
+                                <ColumnDefinition Width="16"/>   <!-- gutter for scrollbar -->
                             </Grid.ColumnDefinitions>
 
                             <!-- TODO GH#6644: Add Icon to command palette entries, in column 0 -->