[2/2] feat: context menu for Keyboard Layout applet
It's in parity with legacy widget.
Flags in the menu are not supported yet.
Counterpart commit with the same title is in P-W
Other than just feature parity, this menu turned out to be quite useful feature - one could have a quick look of all the layout configured, with full names, and if there are more than two of them - it might be easier to switch by selecting in the menu instead of iterating consequentially.