IME: prevent virtual keyboard from unintentionally releasing modifiers (#1721)

IME: prevent virtual keyboard from unintentionally releasing modifiers (#1721)

When Fcitx5 is activated, it creates a virtual keyboard to send keycodes to
applications, then creates a keyboard grab to capture keycodes the user typed.

Before this commit, we set keyboard grab's modifiers to that of currently
active keyboard, which is the virtual keyboard created in the case described
above. However, since the modifiers of the virtual keyboard is empty at first,
we actually set empty modifiers, even when the user is pressing modifiers.
Then, Fcitx5 assumes no modifiers is pressed and redirect the modifier state
back to the compositor via the virtual keyboard. As a result, when the focus
is switched between windows by workspace-switcher, the workspace-switcher is
immediately terminated.

To fix this issue, with this commit, the modifier state of the currently active
keyboard is not set to the keyboard grab if the keyboard is a virtual keyboard
created by the same input-method client.

Fcitx5's commit below is also required to fix the issue.
https://github.com/fcitx/fcitx5/commit/b2924bd361680c493463d240a375b3f0948ae48d