don't focus buttons on click

This prevents the navbar buttons from gaining focus when clicked.
This way it prevents losing focus from the editor or terminal.
It also prevents the settings button from being highlighted after
the settings menu is closed.

src/components/ActionButton.tsx

@@ -51,6 +51,7 @@ class ActionButton extends React.Component<Props> {
                 <Button
                     ref={this.buttonRef}
                     intent={Intent.PRIMARY}
+                    onMouseDown={(e) => e.preventDefault()} // prevent focus
                     onClick={(): void => this.props.onAction()}
                     disabled={this.props.enabled === false}
                     className="no-box-shadow"

src/components/OpenFileButton.tsx

@@ -114,6 +114,7 @@ class OpenFileButton extends React.Component<Props> {
                                         ? { pointerEvents: 'none' }
                                         : undefined
                                 }
+                                onMouseDown={(e) => e.preventDefault()} // prevent focus
                                 // onClick={this.props.onClick}
                                 // breaks Dropzone when this.props.onClick is undefined
                                 // so we have to do it the long way