Properly handle mouse in read-only mode

Summary:
* Do not send mouse events and emulated up/down keys to programs when
   read-only mode is enabled.
* Set the currentTerminalDisplay before sending key events, like in
   keyPressEvent.
* wheelEvent code is restructured a bit to easily separate code not
   allowed in read-only mode.

Test Plan:
* Start Konsole
* Run `man man`, do not press any key after enter
* Turn on read only mode
* Try to scroll using mouse wheel.
* Expected result: scrolling does not work in read-only mode
* Actual result: scrolling works in read only mode (but only when no key
   has been pressed before on 2nd terminal screen)

* Start konsole
* Run a program which uses mouse events, e.g. `vim`
   (enable mouse with `:set mouse=a`)
* Make the program state suitable for using mouse events, e.g. enter a few
   lines of text
* Confirm that mouse events (clicks, selection, wheel) work in the program
* Turn on read-only mode
* Try:
  * Click (left/right/middle button), double click
  * Mouse wheel
  * Click and move (selecting)
* Expected result: mouse events are not passed to the program in read-only
   mode; nothing happens
* Actual result: mouse events are passed to the program

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11763