Put everything in Context behind the same Mutex

[emilk]

Closes #1005

Rebased and continued from #1035

Most of the work was done by @danielkeller

• Everything in a Context is now behind the same mutex.
• ⚠️ Context::input and Ui::input now locks a mutex. This can lead to a dead-lock is used in an if let binding!
  • if let Some(pos) = ui.input().pointer.latest_pos() and similar must now be rewritten on two lines, or with added {} around the righ-hand-side.
  • Search for this problem in your code using the regex if let .*input.
• Context::fonts and Ui::fonts now locks a mutex.
• Renamed CtxRef to Context.
• Context can now be cloned and stored between frames.