IncrementalSearchBox: do not propagate unhandled keys to parent

Summary:
When the focus is in `IncrementalSearchBar`, unhandled keys (i.e. those
not handled by text input box or `unhandledMovementKeyPressed` signal)
are sent to a program running in the terminal, which is probably not
what user expects. Example of propagated keys: Up, Down, PgUp, PgDown,
Fn, Tab.

This patch disables propagation. Shortcuts, movement keys, and keys
handled by search bar still works.

Test Plan:
* Run `cat`
* Open search box
* Press Up/Down/PgUp/PgDown/Tab/Fn keys

Actual result: keys are sent to `cat`
Expected result: nothing should happen (implemented)
Alternative expected result: focus should move from text input to first
button

**Regression test**

Following keys/shortcuts should work:

* Shift+Up/Down/PgUp/PgDown - terminal contents scrolling
* Ctrl+Shift+T - open new tab (unless it was changed)
* Enter - search next
* Esc - close search box

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D14037