Exit fullscreen browse mode with Escape

Summary:
Previous versions of gwenview used ⎋ to exit fullscreen mode.
By changing shortcut handling of ⎋ this does not work anymore.
This patch adds an eventFilter() to leave fullscreen from browse
mode with ⎋.

BUG: 305659

Test Plan:

1. Open gwenview in browse mode
2. Switch to fullscreen mode (e.g. F11)
3. Press ⎋

-> Should switch back to windowed browse mode

Reviewers: ngraham, rkflx

Reviewed By: rkflx

Differential Revision: https://phabricator.kde.org/D10182