keyboardManager: Preserve current keymap across reloads

keyboardManager: Preserve current keymap across reloads

The IM can pretty much update the input sources anytime (even if
to set the same ones). That ends up triggering rebuilding all user
defined keymaps, and losing modifier state if we are unfortunate
enough that this caught us while pressing one.

One common situation seems to be password entries, resulting in
the wrong character being printed if the first character happens
to require the shift key.

If the current keymap is not found in the newly loaded list,
this._current will end up null, with the same behavior as we get
currently (immediate keymap reload).

https://bugzilla.redhat.com/show_bug.cgi?id=1569211

https://gitlab.gnome.org/GNOME/gnome-shell/issues/240

Closes: #240