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 than we get currently (immediate keymap reload).
Closes: #240 (closed)