Add last action status indicator

[jieyouxu]

Closes #3.

[trumank]

Nice! Couple suggestions:

• I would display no status at all before any action has been taken, and also clear the status as soon as an action begins. Seeing "integration complete" while integration is currently happening is a bit confusing.
• The fully saturated green and red status background is also quite harsh so I think LIGHT_GREEN and LIGHT_RED looks better.

Future thoughts

• Add another progress state: "in progress:"
  • Would greatly simplify the "is any action currently occuring" logic
  • Let the action provide status updates e.g. "resolving mod 2 of 3: https://mod.url"
• Add hover text or a popup window to display the entire error message if one occurs.
• Show a checkmark or cross instead of the large "STATUS" text. I spent 30 minutes on it and couldn't get a satisfying look so I'm content with how it is now.

[jieyouxu]

Changes:

• Added LastActionStatus::{Idle, Success, Failure} to handle no-last-action case and status clearing.
• Use LIGHT_GREEN and LIGHT_RED for STATUS instead.