Support compose key in xkbcommon integration

Summary:
The Xkb class now creates a compose key table and a state object and
feeds all key presses through the compose state machine.

Xkb now tracks the latest keysym which is provided through new method
currentKeysym. This is now used when creating a QKeyEvent instead of
passing the key code to the xkb state. With that the keysym can also
be updated through the compose state system.

This only affects KWin internal usage where text is composed, e.g. the
present windows effect filter. Wayland clients do not gain compose key
support, though.

Minimum xkbcommon version raised to 0.5 as compose key support is new
in that version.

Test Plan: Enabled compose key support in keymap and verified through DebugConsole

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2622