Fixes for v1 release

[ethanmoffat]

Thanks @Cirras for testing! This fixes the following discovered issues:

• Quick repeated click events not activating
  • cause: DoubleClick event was suppressing normal clicks
  • fix: Use MouseDown events for all clicks instead of MouseClick (see next bullet)
• Vanilla client uses MouseDown while EndlessClient uses MouseClick
  • fix: Implement detection of MouseDown, replace MouseClick event handlers with MouseDown handlers
• ESC doesn't clear input boxes
  • fix: Implement KeyPress/KeyRelease event handling, detect Esc key via base ClearableTextBox class in EndlessClient
• Selecting character via keyboard shortcut doesn't play a sound
  • fix: Add sfxPlayer call to keyboard shortcut
• Returning to main menu stops background music
  • fix: Only stop background music on transition from PlayingTheGame state
• Unable to get past character select screen
  • fix: TBD
  • improved error handling for async tasks and pop up friendly error message dialog with GitHub report link