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.

• 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