[Feature] Show current mode when switching mode on popup

[naokiri]

The recent change (fcitx/fcitx5#1071 ) seems useful for showing the mode change like AquaSKK.

When switching the mode, show the popup of the switched mode string.

[naokiri]

The API interface shall be added in next version (next of 5.1.10, hopefully 5.1.11) so adding this feature shall require the fcitx5 of that version.

I would keep eyes on Debian/Ubuntu installer official version and add this after that.