save default keyboard layout

Implemented for Global, Virtual Desktop and Application layout policies.
Not implemented for Window policy due separate windows do not preserve their IDs between sessions (still could be implemented the same way as for Application policy).

Layout saving/restoring happens on Session save/load. Covered by unit tests now:

10:53:15: Starting /home/bam/kde/build/kwin/bin/testKeyboardLayout_waylandonly ...
********* Start testing of KeyboardLayoutTest *********
Config: Using QtTest library 5.14.2, Qt 5.14.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 10.0.1 20200328 (Red Hat 10.0.1-0.11))
PASS   : KeyboardLayoutTest::initTestCase()
PASS   : KeyboardLayoutTest::testReconfigure()
PASS   : KeyboardLayoutTest::testChangeLayoutThroughDBus()
PASS   : KeyboardLayoutTest::testPerLayoutShortcut()
PASS   : KeyboardLayoutTest::testDBusServiceExport()
PASS   : KeyboardLayoutTest::testVirtualDesktopPolicy()
PASS   : KeyboardLayoutTest::testWindowPolicy()
error in client communication (pid 128769)
PASS   : KeyboardLayoutTest::testApplicationPolicy()
PASS   : KeyboardLayoutTest::testNumLock()
PASS   : KeyboardLayoutTest::cleanupTestCase()
Totals: 10 passed, 0 failed, 0 skipped, 0 blacklisted, 26613ms
********* Finished testing of KeyboardLayoutTest *********
10:54:18: /home/bam/kde/build/kwin/bin/testKeyboardLayout_waylandonly exited with code 0

Original BUG: https://bugs.kde.org/show_bug.cgi?id=412101