GUI: Display mempool total tx size alongside memory usage in Node window

This answers the ultimate question of life, the universe, and everything: does the mempool fit in the next block?

Also:

• Total tx size and memory usage are now formatted with GUIUtil::formatBytes, rather than separate formatting logic.
• GUIUtil::formatBytes now displays one decimal when using megabytes (previously no decimals).
• Includes bugfix: GUIUtil::formatBytes now uses the appropriate decimal separator for the current language, instead of always a dot (bug existed since !697 (merged) which introduced two decimals when using gigabytes).
• Created a unit test for GUIUtil::formatBytes.

Test plan: try the GUI (open Node window in different languages with different decimal points), and ninja check

Edited by Calin: Note that this MR also affects the way that the bytes are formatted in the following places:

• Peers tab in the Node window
• Network Traffic tab in the Node window

See screenshots: