Screen flicker when updating throughput values in menu bar

The menubar jumps between displaying on a single line and breaking across 2 lines - without any user interaction.

## Expected Behavior

Screen should not flicker unnecessarily. We should reserve sufficient screen space for a reasonable range of throughput values.

## Current Behavior

When throughput values are updated, menu bar briefly breaks across 2 lines.


## Steps to Reproduce (for bugs)

1. Stop all inputs except for a random input, that sleeps 1000ms between messages.
2. Resize the GL window so that menu bar is just shy of breaking lines
3. Observe how it toggles between 1 and 2 lines, when throughput is updated

## Your Environment

* Graylog Version: 6.1